Biology of the dolphinfish ( Coryphaena hippurus ) in the western central Atlantic : a review *

The dolphinfish (Coryphaena hippurus) is one of relatively few circum-tropical oceanic pelagic species. Dolphinfish are found in tropical and subtropical surface ocean water apparently restricted to waters warmer than 20oC (Gibbs and Collette, 1959). They are of significant economic importance throughout their global distribution. In the western central Atlantic they have had a long tradition of seasonal importance to the sports and commercial fisheries of many countries (e.g. Collette, 1978; Palko et al., 1982; FAO, 1994; NMFS, 1996; see also Table 1). Despite their wide distribution and economic importance, they have been the subject of relatively few biological studies in the western central Atlantic (Mahon, 1996) and species-specific management is virtually non-existent in most of the region. Management is currently restricted to minimum size and bag limits in the state waters of Florida and North Carolina, and is now being considered for federal waters (R. Nelson pers. comm., see also SAFMC, 1997). A comprehensive review of biological data available on dolphinfishes (Coryphaenidae) was undertaken by Palko et al. (1982). Here, that review is partially updated by examining the biological data currently available for Coryphaena hippurus (subsequently referred to simply as dolphinfish) in the western central Atlantic, which is considered to be of direct relevance to fisheries management. SCI. MAR., 63 (3-4): 277-301 SCIENTIA MARINA 1999


INTRODUCTION
The dolphinfish (Coryphaena hippurus) is one of relatively few circum-tropical oceanic pelagic species.Dolphinfish are found in tropical and subtropical surface ocean water apparently restricted to waters warmer than 20 o C (Gibbs and Collette, 1959).They are of significant economic importance throughout their global distribution.In the western central Atlantic they have had a long tradition of seasonal importance to the sports and commercial fisheries of many countries (e.g.Collette, 1978;Palko et al., 1982;FAO, 1994;NMFS, 1996; see also Table 1).Despite their wide distribution and economic importance, they have been the subject of relatively few biological studies in the western central Atlantic (Mahon, 1996) and species-specific management is virtually non-existent in most of the region.Management is currently restricted to minimum size and bag limits in the state waters of Florida and North Carolina, and is now being considered for federal waters (R. Nelson pers.comm., see also SAFMC, 1997).
A comprehensive review of biological data available on dolphinfishes (Coryphaenidae) was undertaken by Palko et al. (1982).Here, that review is partially updated by examining the biological data currently available for Coryphaena hippurus (subsequently referred to simply as dolphinfish) in the western central Atlantic, which is considered to be of direct relevance to fisheries management.

Adults
In the western central Atlantic, dolphinfish have been recorded as far north as George's Bank, Nova Scotia (Vladykov and McKenzie, 1935;Tibbo, 1962) and as far south as Rio de Janeiro, Brazil (Ribeiro, 1918;Shcherbachev, 1973).However, it is generally considered to be common only from North Carolina, throughout the Gulf of Mexico and Caribbean to the northeast coast of Brazil, and is only seasonally abundant at these locations (see Table 1).
Even though landings of dolphinfish reported to the Food and Agricultural Organisation (FAO) within the western central Atlantic significantly underrepresent actual landings of this species (Oxenford, in press;Mahon, 1999), they still indicate that dolphinfish are among the top seven oceanic pelagic species landed in this region, giving an indication that they are indeed abundant.There have been no attempts to estimate actual abundance, but timeseries of catch per unit effort data for dolphinfish are mm SL) have been reported during spring, summer and fall (Gibbs and Collette, 1959).Off North Carolina, young dolphinfish have been reported in March and May (Anderson et al., 1956a,b), in late summer (La Monte, 1952;Beardsley, 1967) and in October (Anderson and Gehringer, 1957).
Juvenile dolphinfish (100-400 mm SL) have been reported in the Florida Current in all seasons, but appear particularly abundant in early summer (e.g.Gibbs and Collette, 1959;Beardsley, 1967).
In the Gulf of Mexico, distribution of larval dolphinfish has been described by several authors (e.g.Powles, 1981;Richards et al., 1984;Kelley et al., 1986;Ditty et al., 1994) (Fig. 1).They are apparently present in the Gulf from at least April through to November, and are found in shelf and oceanic waters, although more commonly in the latter (Ditty et al., 1994).Most occurred in water temperatures at or greater than 24 o C (range: 21.4 -32 o C) and in salinities at or greater than 33‰ (range: 18.7 -37.8‰).Furthermore, particularly high abundance was reported near the Mississippi River delta (Ditty et al., 1994).Off Texas, young dolphinfish have been reported in the summer (Pew, 1957;Springer and Pirson, 1958), and Gibbs and Collette (1959) report juveniles in the Gulf of Mexico in spring and summer.
Larval dolphinfish have been reported off Barbados year-round (Lao, 1989), and both larval and juvenile dolphinfish have been sampled in the southeastern Caribbean waters during April/May (Oxenford et al., 1995;Hunte et al., 1995).

Population sex ratio
There are several references to sex ratio of dolphinfish catches from different locations in the western central Atlantic, and these are summarised in Table 2. Generally, females outnumber males in the catch, but sex ratios do appear to differ with size of fish (Rose and Hassler, 1974;Oxenford,1985) and with season (Oxenford, 1985) (Table 3).
The tendency to female biased sex ratios is believed to result from inadvertent selection for females by fishers as a result of intersexual differences in behaviour of dolphinfish, rather than a real difference in sex ratio at conception or in larval and juvenile mortality rates of males and females (Nakamura, 1971;Rose and Hassler, 1974;Oxenford, 1985).Oxenford (1985) suggested that small-sized males and all sizes of females spend more time associated with floating objects than large-sized males, which tend to spend more time in open water, possibly travelling between female dominated schools below rafts.Hence catches of small-sized fish are likely to have a sex ratio approximating 1:1, whilst catches of large-sized fish will be female biased if taken in association with floating objects or male biased if taken in open water.Observations reported by Perez et al. (1992) support this suggestion.Likewise, the slight male bias in the sex ratio reported by Gibbs and Collette (1959) could have resulted from the fact that the majority of their samples were taken by subsurface long-line gear and not from around floating objects.

Description of maturity stages
Maturity stages of male and female dolphinfish in the western central Atlantic have been described by several authors (e.g.Beardsley, 1967;Oxenford, 1985;Perez et al., 1992;Table 4).It is clear from the similarity of these descriptions that dolphinfish are relatively easy to classify into well defined maturity stages based on a combination of visual observation of the gonads and egg size distributions in females.
For dolphinfish from the Florida Current, Beardsley (1967) described 5 maturity stages (I-immature, II-early maturing, III-late maturing, IV-ripe, Vspent) for females; and 2 stages (I-immature or resting, II-mature) for males, based on visual appearance, and also provided examples of egg size distributions (Table 4a).It was noted that no running ripe fish were observed.
For dolphinfish from the Gulf of Mexico, Bentivoglio (1988) used the 5 maturity stages for females and two maturity stages for males as described by Beardsley (1967) (Table 4a).
For dolphinfish from Barbados, Oxenford (1985) also described 4 maturity stages (I-immature, IImaturing, III-mature, IV-spent) for females, and 2 stages (I-immature, II-mature) for males, noting that fish in running ripe condition were not observed, presumably because this state occurs rapidly and only during the pairing and spawning process.It was also noted that spent males could not be differentiated from mature males.A description of each maturity stage based on visual appearance and an example of the typical egg size distribution for each stage is given in Table 4c.-Description of the gonads at each maturity stage for male and female dolphinfish (Coryphaena hippurus) from (a) Florida Current (after Beardsley, 1967), (b) Puerto Rico (after Perez et al., 1992), and (c) Barbados (after Oxenford, 1985).
(a) Florida Current Dolphinfish

Age and size at maturity
Several authors have provided size and/or age at maturity data for dolphinfish from the western central Atlantic (Beardsley, 1967;Schekter, 1982;Oxenford, 1985;Bentivoglio, 1988;Perez et al., 1992;Table 5).Whilst there are differences in both the age and size of dolphinfish at first maturity from different locations, there is general agreement that all dolphinfish in the western central Atlantic reach sexual maturity in the first year of life, and that females reach maturity at a smaller size but similar age to males (Table 5).
In the Florida Current, Beardsley (1967) reported that female dolphinfish begin to mature (reach stage II) at about 350 mm FL (about 6-7 months old), at 450 mm FL 50% are mature, and at 550 mm FL 100% are mature, whilst males mature at a slightly larger size (427 mm FL) than females.Schekter (1982) reported first spawning in laboratory reared dolphinfish from the Florida Current at 6 1/2 months old and at an average weight of 2.5 kg (5 65 mm FL).
In Puerto Rico, Perez and Sadovy (1991) and Perez et al. (1992) reported that the smallest mature female observed was 400 mm FL (384 mm 282 H.A. OXENFORD TABLE 4. (Cont.)-Description of the gonads at each maturity stage for male and female dolphinfish (Coryphaena hippurus) from (a) Florida Current (after Beardsley, 1967), (b) Puerto Rico (after Perez et al., 1992), and (c) Barbados (after Oxenford, 1985).SL), but cautioned that more smaller fish need to be examined to more accurately determine the minimum size at maturity.All fish larger than 600 mm FL were found to be mature, i.e. at stage II or more(Fig.2a).

SEX
In the Gulf of Mexico, Bentivoglio (1988) reported early maturing (Stage II) females as small as 275 mm FL (2 months old).However, not until Stage III (late maturing) were at least two distinct size classes of eggs apparent.He therefore concluded that females reach first maturity between 490 and 520 mm FL (3-4 months old).The smallest mature (Stage II) male was 528 mm FL (4 months old).
In Barbados, Oxenford (1985) provided length frequency distributions for male and female dolphinfish at each maturity stage (Fig. 3).Females were considered to have reached first maturity when a group of large translucent eggs could be clearly distinguished with the naked eye from the mass of smaller pale orange undeveloped eggs in the ovaries (i.e.Stage II gonads), and males were also considered to have reached first maturity when the testes appeared swollen and soft (Stage II gonads).Females were reported to mature at a smaller mini- Batch fecundity-fork F Y~2.52•10 -4 X 3.12*2 ---Y=6.03•10 - X 3.98 -Y=2.7•10 - X 3.67 length relationship (Y=aX b ) Y is no.mature eggs X is mm FL mum size (610 mm SL or 667 mm FL) than males (735 mm SL or 805 mm FL), but at approximately the same age (112 days for females, 108 days for males).By 5 1/2 months (850 mm SL or 931 mm FL for females and 1074 mm SL or 1178 mm FL for males) 99% of fish were reported to be fully mature (Fig. 2b).

Gonasomatic indices
Limited gonasomatic index (GSI) data are available for dolphinfish from the western central Atlantic.GSI values for mature individuals have only been reported from Barbados, and range from 1.02 to 7.90% for mature (Stage II and III) females.For mature (Stage II) males they are considerably lower, ranging from 0.19 to 0.48% (Oxenford, 1985).Mean GSI values at each maturity stage for both sexes are also given by Oxenford (1985) for dolphinfish from Barbados (Fig. 4).Population monthly mean GSI values for both sexes are available for dolphinfish from Puerto Rico (Perez et al., 1992) and from Barbados (Oxenford, 1985) (Fig. 5).Puerto Rico dolphinfish appear to have higher GSI values than Barbados dolphinfish, and show a different seasonal pattern.

Fecundity and egg size
Dolphinfish from the western central Atlantic typically have two or three size classes (batches) of eggs in the ovaries: one heterogeneous size class of small eggs, and one or two more homogeneous size classes of larger maturing or mature eggs (Beardsley, 1967;Oxenford, 1985;Perez et al., 1992;Fig. 6).Mean mature egg size appears to vary slightly with location and/or with author, ranging from 0.97 to 1.10 mm diameter (Table 5).Hassler and Rainville (1975) estimated dolphinfish eggs from North Carolina to be approximately 1.3 mm diameter, and Ditty et al. (1994) reported a mean size of 1.4 mm diameter from the Gulf of Mexico.However, it should be noted that these apparently larger eggs were collected from the plankton rather than from the ovaries of ripe fish.
Batch fecundity estimates for dolphinfish in the western central Atlantic range from 58,000 to 1.5 million (Table 5) and are strongly influenced by fish size.Batch fecundity -length relationships are avail-able for dolphinfish from Florida, Puerto Rico and Barbados and all show an exponential increase in egg number with fish size, the exponent being between 3 and 4 (Table 5, Fig. 7).Dolphinfish from the Florida Current and Puerto Rico appear to have very similar fecundity-size relationships, whilst Barbados dolphinfish appear to be less fecund at size.

Spawning season and location
There are numerous references to time of spawning for dolphinfish in the western central Atlantic (e.g.Palko et al., 1982) which clearly show protracted multiple spawning behaviour.The presence of several size classes of eggs in the ovaries indicates that they are batch spawners and probably spawn at least two or three times in each spawning period (Beardsley, 1967;Oxenford, 1985;Perez and Sadovy, 1991).Schekter (1982) reported almost continous spawning from dolphinfish brood stock captured from the Florida Current and held in captivity for several months.
Off North Carolina, spawning dolphinfish have been reported in May and June (Schuck, 1951), dolphinfish eggs have been collected in July and August (Hassler and Rainville, 1975), and peak spawning is reported to occur during June and July (Rose, 1966).Relationship for Florida Current was obtained by extrapolation of graphical data given by Beardsley (1967).Relationships for Puerto Rico and Barbados dolphinfish were given by Perez et al. (1992) and Oxenford (1985) repectively.
In the Florida Current, the presence of very young dolphinfish throughout most of the year suggests that dolphinfish spawn there almost yearround (Gibbs and Collette, 1959;Beardsley, 1967;Shcherbachev, 1973;Fahay, 1975;Powles and Stender, 1976).Beardsley (1967) reports a spawning season from November to July with a peak in spawning activity from January to March (Fig. 8a).
In Puerto Rico, ripe females occur throughout much of the year (September-June; Perez and Sadovy, 1991;Perez et al., 1992; Fig. 8b), although peak spawning events appear, from mean GSI data, to occur in March and in June (Fig. 5a).
In the Gulf of Mexico, the presence of small dolphinfish larvae year-round suggests that dolphinfish are spawning all year in the south and at least from April to December in the northern Gulf, with possible peaks in the spring and early fall (Ditty et al., 1994).
In Barbados ripe and spent fish are reported to occur in all months that the dolphinfish fishery is active (November-June) and peak spawning appears, from mean GSI data, to be from May through June or possibly longer (Oxenford, 1985) (Table 6, Fig. 5b).Larval dolphinfish occur off Barbados in all months and are most common from February to May (Lao, 1989).
Location of dolphinfish spawning in the western central Atlantic is poorly documented, but presumably widely spread, based on reports of the location of ripe fish and small larvae from the southeastern USA, Gulf of Mexico, Puerto Rico and Barbados (see above).Ditty et al. (1994) infer from the distribution of very small (less than 7 mm) larvae in the Gulf of Mexico that spawning occurs in the oceanic waters of the Gulf rather than on the shelf there.Oxenford and Hunte (1986a) contend that maximum spawning by the proposed northeastern and southeastern Caribbean dolphinfish populations will occur when the dolphinfish are large.For the northeastern stock this will be in the vicinity of Puerto Rico at the most up-current limit of their proposed range.For the southeastern stock maximum spawning is also proposed to occur at the most up-current limit of the migration circuit, off the north coast of Brazil (Oxenford and Hunte, 1986a).
REVIEW OF DOLPHINFISH BIOLOGY 287 locations (Table 8), there is general agreement that dolphinfish in the western central Atlantic grow extremely fast (first year growth estimates for wild fish range from 1.43 to 4.71 mm d -1 ) and have an average longevity of less than 2 years.In North Carolina, Rose and Hassler (1968) examined scales for annuli in 738 dolphinfish.They found 593 0-group fish (size range: 400-725 mm FL, mean: 572 mm FL), 117 I-group fish (size range: 650-1100 mm FL, mean: 868 mm FL), 20 IIgroup fish (size range: 900-1300 mm FL, mean: 1108 mm FL), and 8 III-group fish (size range: 1100-1430 mm FL, mean 1269 mm FL).A mean first year growth rate of 1.56 mm FL d -1 is inferred from these data.Rose and Hassler (1968)  phinfish, but noted that 96% die before they are 2 years old.There are also growth rate data for dolphinfish reared in captivity from North Carolina brood stock indicating early juvenile growth of 1.07 mm TL d -1 (Hassler and Rainville 1975) and approximately 5.88 mm SL d -1 (Hassler and Hogarth 1977).Von Bertalanffy growth curve parameters were estimated by Rivera Betancourt (1994) for North Carolina dolphinfish using data from Rose and Hassler (1968) and are given in Table 9.
In Florida, Beardsley (1967) examined scales for annuli in 511 dolphinfish.He found 121 I-group fish (size range: 400-1175 mm FL, mean: 725 mm FL), 9 II-group fish (size range: 1025-1325 mm FL, mean: 1175 mm FL), 1 III-group fish (1425), and 1 IV-group fish (1525 mm FL).A mean first year growth rate of 1.99 mm FL d -1 is inferred from these data (Table 8).Beardsley (1967) noted that males grow faster and appear to reach larger maximum size than females.He suggested that the maximum life span for Florida dolphinfish was 4 yr, although most (98%) die before they are 2 years old.There are also growth rate data for small numbers of dolphinfish held or reared in captivity from wild caught Florida fish or Florida brood stock indicating daily growth rates of approximately 2.65 mm FL for a single fish held in the Miami Seaquarium (Beardsley, 1967); 2.73 mm SL for F 1 generation fish (Schekter, 1982); 3.91 mm FL for 2 fish held at the Florida Marineland Marine Studios (Herald, 1961;cited by Beardsley, 1967); 9.66 mm SL for 4 wild females held as brood stock (Schekter, 1982); and between 2.67 and 3.98 g (over 290 H.A. OXENFORD  Murray (1985) 80 and 150 days respectively) for Florida reared fingerlings held in sea pens off the northwest coast of Barbados (Prescod and Hunte 1997) (Table 8).Von Bertalanffy growth curve parameters were estimated by Pauly (1978) and Rivera-Betancourt (1994)for Florida fish using data provided in Beardsley (1967) and are given in Table 9.
In Puerto Rico, Rivera Betancourt (1994) used daily growth checks in the sagittal otoliths of dolphinfish from 550 to 1325 mm FL, and reported an average first year growth rate of 2.52 mm FL d -1 (Fig. 11, Table 8).Differential growth rates between the sexes was also reported by Rivera Betancourt (1994) for Puerto Rico dolphinfish, with males growing faster (2.54 mm FL d -1 ) than females (2.46 mm FL d -1 ) during their first year (Fig. 11).Von Bertalanffy growth curve parameters were also provided by Rivera Betancourt (1994) for all Puerto Rico dolphinfish and for males and females separately (Table 9).
In the Gulf of Mexico, Bentivoglio (1988) used presumed daily growth checks in sagittal otoliths of dolphinfish and reported an average first year growth rate of 3.88 mm SL d -1 (4.15 mm FL d -1 ) for fish of size range 250-1200 mm SL (Fig. 12), and 0.49 mm SL d -1 (0.61 mm FL d -1 ) for large-sized fish (850-1200 mm SL).Bentivoglio (1988) reported a longevity of less than 1 yr.Von Bertalanffy growth curve parameters were also provided for dolphinfish from the Gulf of Mexico by Bentivoglio (1988) and by Rivera Betancourt (1994) using Bentivoglio's data (Table 9).
In the eastern Caribbean, Oxenford and Hunte (1983) did not detect any scale annuli in 558 dolphinfish from Barbados, but were able to read presumed daily growth checks in the sagittal otoliths of dolphinfish from 174-1100 mm SL, inferring an overall average first year growth rate of 4.71 mm SL d -1 , and an average growth rate in large fish (600-   Murray (1985) 1200 mm SL) of 1.43 mm SL d -1 (Table 8, Fig. 13a).
Monthly progression of the mean size of dolphinfish landed by the commercial fishery in Barbados indicated a similar growth rate (1.53 mm SL d -1 ) for large fish (600-1200 mm SL) (Fig. 14).Differential growth rates between the sexes was also reported by Oxenford (1985) for Barbados dolphinfish, with males growing faster (5.77 mm SL d -1 ) than females (4.23 mm SL d -1 ) during their first year (Fig. 13b).Oxenford (1985) found no fish older than one year and concluded that Barbados dolphinfish are essentially annual, living to a maximum of around 14 months.Von Bertalanffy growth curve parameters and growth performance factor (P; from Pauly, 1979) were also provided by Oxenford (1985) for all Barbados dolphinfish and for males and females separately (Table 9, Fig. 15).Murray (1985) used the probability paper method of Cassie (1954) to examine monthly progression of size frequency data for dolphinfish landed by the commercial fishery in St. Lucia, and concluded that it was possible to follow five growth trajectories showing growth rates of between 32 and 81 mm per month.These data infer a mean growth rate for adult dolphinfish (693-1674 mm FL) from St. Lucia of 1.78 mm FL d -1 (Table 8).Murray (1985) also provided von Bertalanffy growth curve parameters from these data (Table 9).

Mortality rates and longevity
Athough longevity estimates have been made by several authors (see above), there are only three studies which report mortality estimates for dolphinfish from the western central Atlantic (Oxenford, 1985;Murray, 1985;Bentivoglio, 1988;   For Gulf of Mexico dolphinfish, Bentivoglio (1988) used the Robson and Chapman (1961) least squares method to estimate mortality rates from otolith-aged specimens using various time intervals to obtain a range of total instantaneous mortality values (Z) between 8.18 and 8.67, which suggest actual annual mortality rates of between 99.97 and 99.98% ( For eastern Caribbean dolphinfish, Oxenford (1985) used the size structure of dolphinfish taken by the commercial pelagic fishery in Barbados (Fig. 16a) and size-at-age data (Fig. 15a) to determine the size/age at which dolphinfish become fully vulnerable to the fishery (775 mm SL, 4 mo), and to construct a catch curve (Fig. 16b) and estimate an annual instantaneous total mortality (Z = 3.9).Oxenford (1985) also provided alternative estimates of Z using: the relationship of Beverton and Holt (1956) and an average size of 937 mm SL for fully vulnerable fish; and the relationship of Hoenig (1983) and an estimated t max of one year (Table 10).All estimates predict an actual total annual mortality (A) of between 98.0 and 99.7% (Table 10).Oxenford (1985) also provided estimates of natural mortality for dolphinfish from Barbados, using the empirical formula of Pauly (1980), a mean water temperature of 28 o C and von Bertalanffy parameters for all fish (L oo = 155.9cm TL, annual k = 3.49) (Table 10).Murray (1985) used the catch curve method of Ziegler (1979) to estimate an annual instantaneous total mortality (Z = 3.53) for dolphinfish from St. Lucia, predicting an actual total annual mortality (A) of 97.1% (Table 10).Murray (1985) also provided an estimate of natural mortality for dolphinfish from St. Lucia, using the empirical formula of Pauly (1983), a mean water temperature of 27.5 o C and von Bertalanffy parameters for all fish (L oo = 236.1 mm TL, annual k = 0.532) (Table 10).
The estimates of total mortality for eastern Caribbean dolphinfish are very similar.However, the natural mortality estimates differ markedly, as a result of using an empirical formula dependent on very different estimates of growth curve parameters for Barbados and St. Lucia dolphinfish.Given that the growth curve for Barbados dolphinfish was estimated from size-at-age using otolith increments over a wide size range of fish (174-1100 mm SL), and the St. Lucia growth curve was estimated from monthly progression of size frequencies of adult fish only (693-1674 mm FL), the former is likely to be more representative.

Diet
Several authors have commented on the diet of dolphinfish from the western central Atlantic (e.g. from the Atlantic: Shcherbachev, 1973;southeast and Gulf coast of the USA: Manooch et al., 1984;North Carolina: Schuck, 1951;Rose and Hassler, 1974;Gulf Stream: Gibbs and Collette, 1959;Barbados: Lewis and Axelsen, 1967;Oxenford, 1985;Oxenford and Hunte, 1999).The relative importance of prey items to dolphinfish reported by these studies is summarised by ranking the top 5 prey items according to frequency of occurrence; numerical abundance; and bulk in Table 11.Larval dolphinfish feeding habits have also been reported by Schekter (1972) who examined 11 specimens (size range 5.8 -37.3 mm) from the Florida Current, and found 146 prey items in their stomachs (85 copepods, 49 invertebrate eggs, 2 fish and 10 unidentified objects).All studies agree that dolphinfish feed on a wide variety of fish and invertebrates, including: juveniles of large oceanic epipelagic species; juveniles and adults of small oceanic epipelagic species; juveniles and adults of mesopelagic species that demonstrate diurnal migrations to the surface; and pelagic larvae and juveniles of neritic benthic species.This suggests that dolphinfish probably forage opportunistically rather than selectively, a feeding strategy that appears to be common among tropical pelagic species.
A comparison of diets by frequency of occurrence of prey items in dolphinfish stomachs is also given in Fig. 17 for dolphinfish from the southeastern and Gulf states, North Carolina, Gulf Stream and Barbados.It is apparent that there are geographical differences in the diet, which probably reflect differences in availability of prey items, rather than differential selection by dolphinfish from different locations.It is interesting that there is a fairly strong similarity in the diets of dolphinfish from Barbados reported by different studies conducted 18 yr apart.
Two studies (Manooch et al., 1984;Oxenford and Hunte, 1999) calculated indices of relative importance (IRI) for prey items of dolphinfish, to give a less biased assessment of the diet, and these are given in Table 12 for dolphinfish from the southeastern and Gulf coasts of the USA, and for Barbados.

MOVEMENTS, MIGRATION AND STOCK STRUCTURE
Dolphinfish are considered to be highly migratory, being only seasonally abundant over most of their range in the western central Atlantic (Table 1).However, for an oceanic pelagic species, it is relatively small (maximum size 200 cm TL and 25 kg), short-lived (essentially an annual species) and tends to approach coastal waters.It is therefore likely to have a more complex stock structure (i.e. a larger number of smaller stocks with more localised migration circuits) than has been proposed for many of the larger highly migratory truely oceanic species such as the marlins, swordfish and large tunas (e.g.yellowfin, bigeye, albacore) which grow to sizes in excess of 100 kg, are long-lived (> 6 yr) and are considered to have between one and three Atlantic-wide stocks (Oxenford, in press).Palko et al. (1982) report that migrations and movements of dolphinfish are likely to be affected by the movement of drifting objects on the high seas, with which they are often closely associated.Direct evidence of dolphinfish movements within the western central Atlantic are very limited.Although tagging programmes have included dolphinfish in the western central Atlantic (e.g. Rose and Hassler, 1968;NMFS, 1996;CFRAMP, pers. comm.), sample sizes have either been very small or results of dolphinfish tag recaptures have not been published in the primary literature.However, several authors have proposed migration hypotheses for dolphinfish.Oxenford and Hunte (1986a) proposed two migration circuits in the northeast and southeast Caribbean, based largely on seasonality of the dolphinfish fisheries by location and mean size at capture.They suggest a northeastern migration circuit incorporating the northern Caribbean islands, the southeastern United States and Bermuda, and a southeastern circuit incorporating the southeastern Caribbean islands and the north coast of Brazil (Fig. 18).Dolphinfish following the northeastern circuit are believed to travel northwestwards from around Puerto Rico in February through the Bahamas in April/May, to Florida and Georgia by May/June, South and North Carolina by June/July, and then onwards in a southeasterly direction into the Atlantic, passing Bermuda in July/August and REVIEW OF DOLPHINFISH BIOLOGY 297  Mather (pers. comm., cited by Rose and Hassler, 1968), who reported 2 tag recaptures showing displacement from the Florida coast 97 km and 260 km northwards.Additional support for this hypothesis is also found in Beardsley (1967), who reported that dolphinfish probably move northward from Florida during spring and summer, and in Gibbs and Collette (1959), who suggested that the spring abundance of dolphinfish in northern Caribbean islands (Virgin Islands and Puerto Rico) may be a prespawning migration, mainly by females.The proposed south to north migration of dolphinfish along the Atlantic coast of the USA is strongly supported in some years by observed patterns of peak occurrence in the sport fisheries (Palko et al., 1982).However, in other years their CPUE data for the sport fishery support a synchronised offshore to onshore movement of dolphinfish, possibly reflecting the differences in the distance of blue water from the shore (Palko et al., 1982).Perez et al. (1992) suggest an alternative westerly migration route for the southeastern dolphinfish population, based on the observation of a second peak in abundance of dolphinfish in Puerto Rico.They suggest that the southeastern population would pass the Virgin Islands and southern Puerto Rico during April/May and migrate either southwest into the Caribbean Sea or westward into the Gulf of Mexico.In the Gulf of Mexico, CPUE data from the sport fishery indicates an offshore-onshore movement of dolphinfish (Palko et al., 1982).

Stock structure
There has been one preliminary investigation of dolphinfish stock structure within the western central Atlantic, which suggests that there are at least two separate unit stocks in the Caribbean Sea, located in the northeast and southeast (Oxenford and Hunte, 1986a).The hypothesis was based on: observed seasonality (months of peak abundance) and mean size of dolphinfish from commercial and sport fisheries (which suggested two different migration circuits; Fig. 18); a comparison of life history characteristics of dolphinfish from North Carolina, Florida and Barbados (which showed marked differences in average first year growth rates, fecundity-length relationships, size and age at first maturity, and mean mature egg size); and observed differences in allelic frequencies at the IDH-2 locus determined through electrophoresis.
Possible alternative hypotheses of (1) a generalised north-south movement of a broadly distributed population, and (2) a seasonal onshore-offshore movement, have been suggested by Mahon and Mahon (1987).However, no alternative stock structure hypothesis has yet been tested.The proposed location of the boundary between the putative contiguous stocks remains unclear.For example, Perez and Sadovy (1991) noted two abundance peaks and differences in the mean size of dolphinfish landed on the north and south coasts of Puerto Rico and suggested that two populations may be present seasonally in Puerto Rico.However, a comparison of reproductive traits between dolphinfish landed on the north and south coasts of Puerto Rico failed to detect differences between them (Perez et al., 1992).Furthermore, a comparison of growth rates of dolphin occurring on the north and south coasts of Puerto Rico failed to find expected differences, and were similar to growth rates of the southeastern stock (Rivera-Betancourt, 1994).Three explanations were proposed by Rivera Bertancourt (1994): (i) that Puerto Rico dolphinfish represent the northern extreme of the southeastern stock, (ii) that Puerto Rico dolphinfish belong to a smaller intermediate stock, or (iii) that age and growth studies for the northern study were flawed and that there is only a single stock.
The existence of additional stocks particularly within the Gulf of Mexico and central/western Caribbean is very likely, but has not been investigated.This remains a high priority issue for resolution, if appropriate stock assessments are to be conducted and management strategies are to be developed for dolphinfish in the western central Atlantic.
laterally compressed (narrow), firm and show some convolutions.Colour may vary with freshness of sample.Most samples collected to date have a dark pink/brown appearance.No milt extrudes when cut or squeezed.Microscopic description: Crypts of spermatogonia and early spermatogenesis develop together around each seminiferous tubule.Gonad small and compact.Few or no sperms are present.II Mature (or ripe) Testes lobules are narrow.They could be small and compact like immature testes or have convolutions.Tissue feels soft.Milt may extrude when cut or slightly squeezed.Colour varies from pale pink to white.Microscopic description: The seminiferous tubules are cysts in several stages of spermatogenesis.Throughout the testes, the cysts have the sinuses partially or totally full of spermatides and/or sperm.Seminiferous tubules closer to the central or efferent ducts are connected and elongated, partially or totally full of spermatozoa.III Spent Testes are flaccid with dark pink/brown colour.They may be confused with immature testes.Microscopic description: Disorganized appearance and elongated cysts.Crypts of spermatocytes and spermatides are present, but most seminiferous tubules and sinuses are empty and some have remnants of sperms.Some spermatogonia may be observed.orange, firm, narrow ovaries.No eggs visible through ovary wall.II Maturing Medium orange, firm, enlarged ovaries.Eggs visible through ovary wall and small blood vessels over surface of ovary.III Mature Pale orange/yellow, soft swollen ovaries.Eggs and large blood vessels visible.IV Spent Pale orange, large flaccid ovaries.Few eggs visible and large blood vessels.Males I Immature Narrow, slightly lobed, firm testes.Colour varies from pale flesh to dark brown/pink.No milt exudes when cut.II Mature Large, highly lobed, soft, fragile testes.Colour varies from pale flesh to dark brown/pink.Milt exudes when cut and squeezed.
FIG. 10. -Comparison of length-weight relationships reported for dolphinfish (Coryphaena hippurus) in the western central Atlantic.(a) shown separately by sex (after Oxenford and Hunte, 1986b), and (b) overall (calculated from data presented by Schuck, 1951 for North Carolina, by Rivera Betancourt, 1994 for Puerto Rico, and by Oxenford, 1985 for Barbados).
FIG. 11.  -Relationship between daily growth increments in sagittal otoliths and fork length for dolphinfish (Coryphaena hippurus) from Puerto Rico.Slope of regression line indicates mean first year growth rate and curve indicates the von Bertalanffy growth curve for (a) all fish, (b) males, and (c) females(after Rivera Betancourt, 1994).
FIG.  12. -Relationship between daily growth increments in sagittal otoliths and standard length for dolphinfish (Coryphaena hippurus) from Gulf of Mexico.Slope of regression line indicates mean first year growth rate for both sexes (afterBentivoglio, 1988).

TABLE 2 .
-Overall sex ratios reported for dolphinfish (Coryphaena hippurus) landings in the western central Atlantic.

TABLE 3 .
-Sex ratios by size and season for dolphinfish (Coryphaena hippurus) landings in the western central Atlantic.

TABLE 5 .
-Summary of reproductive characteristics reported for dolphinfish (Coryphaena hippurus) from the western central Atlantic.

TABLE 6 .
-Percentage of mature male (Stage II) and female (Stage II, III and III) dolphinfish (Coryphaena hippurus) and mean sizes of both sexes observed in the pelagic fishery catch each month inBarbados (after Oxenford,1985).

TABLE 7 .
-Summary of length-weight relationships for dolphinfish (Coryphaena hippurus) from the western central Atlantic given in the form: Wt = aL b , where Wt is weight in kg and L is fork length in mm.

TABLE 8 .
-Summary of first year growth rate estimates for dolphinfish Coryphaena hippurus from the western central Atlantic.For captive or laboratory reared fish the source of the fish is given in parentheses.

TABLE 11 .
Table10).-Dietary importance (by rank) of the five main prey categories of dolphinfish (Coryphaena hippurus) from the western central Atlantic, assessed by (a) numerical abundance, (b) frequency of occurrence in the stomachs, and (c) total bulk (weights, volumes or lengths).

TABLE 12 .
Manooch et al., 1984)ortance of the main prey categories of the dolphinfish (Coryphaena hippurus) from the southeastern and Gulf states of the USA (afterManooch et al., 1984), andBarbados (after Oxenford and Hunte, 1999).IRI -index of relative importance.theVirgin Islands and Puerto Rico again by November/December.Dolphinfish following the southeastern circuit are believed to travel northwards, passing Grenada in February/March, St. Vincent, Barbados, St. Lucia and Martinique in March/April, Dominica and the Virgin Islands in April/May and then onwards in a southeasterly direction into the Atlantic and back down to the northeast coast of South America.Part of the proposed northeastern circuit is supported by FIG. 18. -Proposed migration circuits and locations of putative northeastern and southeastern Caribbean dolphinfish (Coryphaena hippurus) stocks in the western central Atlantic (after Oxenford and Hunte, 1986a).Dark arrows indicate segments of the circuits for which seasonality and size data are available; clear arrows indicate areas where no data are available.reaching