The fishes of the family Triglidae, commonly known as gurnards or sea robins, are medium-sized demersal fishes that are found on soft bottoms in coastal areas up to 500 m depth in all oceans, in both tropical and temperate regions (Hureau 1986, Richards and Jones 2002). Gurnards are commercially important in European waters. In France, they constitute the most important fishery in the western English Channel (ICES Division VIIe), where the market is well established (ICES 2006); in the North Sea, the grey gurnard, Eutrigla gurnardus (Linnaeus, 1758) is used for human consumption (ICES 2014); in UK coastal waters especially in the English Channel, gurnards are targeted commercially (Marriott et al. 2010); and in the Mediterranean, tub gurnard, Chelidonichthys lucerna (Linnaeus, 1758), streaked gurnard, Chelidonichthys lastoviza (Bonnaterre, 1788) and longfin gurnard, Chelidonichthys obscurus (Walbaum, 1792) are a valuable by-catch of mixed demersal fisheries (Colloca et al. 2019). When landed, they are mainly reported as “mixed gurnards” making it difficult to interpret official landings, but an average of 3905 t of gurnards were landed yearly between 2010 and 2017 in European Union-28 (Eurostat 2021). In 2010, legislation in the EU changed, requiring gurnards to be landed under their exact species names (ICES 2014). Except for red gurnard, Chelidonichthys cuculus (Linnaeus, 1758), for which the precautionary approach was applied and landings were recommended not to exceed 2894 t in 2020 and 2021 in ICES subareas 3-8 (ICES 2019), there is currently no technical measure specific to gurnard species in EU waters, and their exploitation is only subject to the general regulation in the areas where they are caught (ICES 2014). Nevertheless, the increasing interest in gurnards as potential commercial species has promoted the development of monitoring programmes for stock assessment, including the one recommended by ICES for C. cuculus within the new Memorandum of Understanding between the European Commission and ICES, aimed at obtaining information on biological parameters (ICES 2006). However, the impact of fishing on gurnard populations has rarely been examined, and studies on key biological parameters and population dynamics used for fisheries management have focused on targeted species, ignoring by-catch or discarded ones, resulting in a lack of biological data and incomplete information about fishing exploitation (Ordines et al. 2014).

This is specifically the case for gurnards off the Portuguese coast, where eight species are caught by the artisanal (mainly polyvalent) and trawler (mainly bottom) fleets: C. cuculus, C. lastoviza, C. lucerna, C. obscurus, E. gurnardus, large-scaled gurnard, Lepidotrigla cavillone (Lacepède, 1801), spiny gurnard, Lepidotrigla dieuzeidei Blanc and Hureau 1973, and piper gurnard, Trigla lyra Linnaeus, 1758. Gurnards are within the top 25 fish species most landed in Portuguese waters, with an average of 371 t landed per year between 2010 and 2019 (INE and DRGM 2011-2020). Among the four most landed species are C. cuculus and T. lyra (Rocha et al. 2018). For C. cuculus, information available on life-history parameters (age, growth and reproduction), abundance and distribution, length and weight relationships, fisheries data, stock identification and management is scattered in European waters (ICES 2006, Marriott et al. 2010, Ordines et al. 2014). For T. lyra, studies on age and growth, length-age composition, reproduction and mortality were published for the Saronikos Gulf (Papaconstantinou 1981) and the Aegean Sea (Papaconstantinou et al. 1992, Ismen et al. 2016).

Key biological parameters routinely used in fisheries to estimate exploitation level (ICES 2018) and still absent from gurnards were analysed in the present study. Therefore, the main goals were (1) to estimate the sex ratio and the weight-length relations, (2) to validate the assignment of age, (3) to model growth, and (4) to estimate mortality and obtain an initial approximation of the exploitation level for C. cuculus and T. lyra in the coastal waters of western Portugal.

This study presents the first age, growth and mortality characterization for T. lyra and C. cuculus from the northeast Atlantic (Portuguese) coast, two by-catch species caught by artisanal and trawler fleets. According to ICES (2014), the quality of landings data is poor since the species have usually not been well separated when landed, and updated biological parameters are lacking, particularly for the most important commercial gurnards such as C. cuculus (ICES 2006). For a correct stock assessment and sustainable management of fisheries, detailed biological information from all the different species is needed.

Individuals of both species were obtained from commercial fishing vessels using the same fishing gear, but the length range varied between species. Samples of C. cuculus included individuals of all length classes between 10 and 43 cm, though most were larger than 23 cm, as expected from landings. Nevertheless, the length distribution gathered from the present study falls within those observed for other European areas (see Table 3). T. lyra individuals under 22 cm were underrepresented, and no individuals between 16 and 22 cm were collected from landings; the few individuals gathered were obtained directly from fishermen. This could be related to species-specific ecological features, as they have differeent bottom type preferences, different feeding patterns, a specific depth distribution (Papaconstantinou 1983, Lopez-Lopez et al. 2011) and contrasting reproductive strategies (Neves et al. 2021), which may influence their selectivity in the fishery. According to Lopez-Lopez et al. (2011), on the Galician and Cantabrian Sea continental shelves, mature T. lyra over 26 cm are restricted to shallower depths (around 150 m), which is suggested to be related to an ontogenetic shift associated with a change in diet. The fact that individuals under 26 cm are underrepresented in the present study can be related to their higher depth distribution, the size less commercially valued, and the selectivity of the gears regarding the size and the depth where they are used on the Portuguese coast (artisanal gears are used in shallow waters).

Females proved to be more frequent in landings than males for both species in the sample, contrary to findings in the Saronikos Gulf for T. lyra (Papaconstantinou 1981) and in the Aegean Sea for C. cuculus (Papaconstantinou 1983), where males dominated. Differences in length and weight between sexes were also found, with females being larger and heavier. This is consistent with records for C. cuculus in northwest Wales and eastern Anglesey (Marriott et al. 2010), although in the Saronikos Gulf no differences in length between sexes were observed for T. lyra (Papaconstantinou 1981).

There were no significant differences in length-weight relationships between males and females, and growth proved to be isometric in both studied species, contrarily to what was found on the south coast of Portugal, where different length-weight relations for both sexes were found, and species showed positive allometric growth (Olim and Borges 2006). This can be related to the lower length range sampled in that study (C. cuculus<15.8 cm and T. lyra<24 cm) covering mainly juveniles (Ismen et al. 2016, Neves et al. 2021). In other geographic areas, C. cuculus showed positive allometric growth with similar length-weight relations between sexes (Olim and Borges 2006, Marriott et al. 2010, Vallisneri et al. 2014). These differences are not unusual and may vary within species depending on the season, population and environmental conditions, which may affect the weight of adults through the maturation of gonads or stomach fullness (Froese 2006). This variation was minimized in the present study by using eviscerated instead of total weight, but it was not possible to account for differences among studies in the length range of individuals.

Edge analysis and MIR validated the annual periodicity of annulus formation in T. lyra and C. cuculus, which is crucial for age and growth studies. The two age-validation methodologies proved that deposition in both species corresponds to one annulus per year with opaque increments formed in spring/summer. A similar pattern was found for C. cuculus in the Aegean Sea (Papaconstantinou 1983) and the Central Tyrrhenian Sea (Colloca et al. 2003) and is likely associated with higher food availability and therefore more energy available for allocation to growth.

In both species, there was a good separation of the first two increments followed by an increasing overlap with distance to the nucleus. This may be associated with energy investment in both growth and reproduction after maturation, leading to narrower annual increments (Wootton 1992). Indeed, length at first maturity estimated for T. lyra off the northern Aegean Sea and C. cuculus off the Portuguese coast varied between 21.9 and 22.7 cm for males (Ismen et al. 2016) and between 19.9 and 22.1 cm for females (Neves et al. 2021), respectively, which according to the present study should correspond to a maturation age of around two years.

Analysis of precision and bias between readers indicated good consistency for both species, with CV and APE values below the limits suggested by Campana (2001). Nevertheless, T. lyra otoliths were more difficult to read than those of C. cuculus, with higher values for APE, CV and D and a lower percentage of total agreement between readers. Otoliths of T. lyra displayed some discontinuities and false rings, making it more difficult to identify the annuli, especially in larger individuals, which attained older ages than C. cuculus, and thus made the ageing process more difficult. But whole otoliths proved to be good structures for evaluating age in these species, without the need for additional treatment, such as burning in T. lyra (Papaconstantinou 1981, Papaconstantinou et al. 1992) and crack and burning in C. cuculus (Baron 1985). The maximum age estimated was 11 years (TL=44.4 cm) for T. lyra and 9 years (TL=42.1 cm) for C. cuculus, values that were slightly above those obtained in other studies (Table 3).

To estimate von Bertalanffy growth parameters, different approaches were needed for the two species. The scarcity of young individuals of T. lyra precluded the adjustment of the von Bertalanffy equation to the age-length key and justified the use of two additional approaches: the back-calculation method and a combined method (Paiva et al. 2013, Gordo et al. 2016). The combined methodology showed the best adjustment to the data and the most realistic growth parameters.

T. lyra attains a lower TL, and K is naturally higher in this species. Differences in estimated parameters for both species from other European waters were obvious and reflected in different values of ɸ’ (Table 3). T. lyra from the Mediterranean showed lower K values, suggesting slower growth rates than in the present study. Results for C. cuculus are more difficult to interpret given the data variability among different studies, but the significant difference in mean ɸ’ between the Atlantic and Mediterranean populations suggests that they may have different growth strategies, as already observed by Marriott et al. (2010). Several factors can contribute to these differences, such as the use of different ageing methodologies, sample heterogeneity, different environmental conditions, latitudes and even fishing pressures.

Mortality is an important parameter to understand population dynamics and one of the most influential parameters in stock assessment. Although evaluation of mortality is of great concern among important exploited target species, this does not always happen for by-catch and low-value species such as gurnards. Given the difficulty of estimating M directly and reliably, two estimators based on one and two parameters were used. As expected, some variation was found depending on the method (Then et al. 2015). For the Portuguese coast, T. lyra fishing mortality is lower than for C. cuculus, also facing a lower exploitation rate. The exploitation rate estimated for northwest Wales and eastern Anglesey (E=0.64 yr^-1, Marriott et al. 2010) is similar to those obtained in the present study which, according to Gulland (1971) are considered high because they are over 0.50 yr^-1. Species-specific time series with information on length composition in commercial landings could allow us to evaluate the evolution in exploitation rates over time and infer its influence on age composition, which was not possible given the absence of information. On the other hand, these results are in line with those of other studies because the official landings statistics indicate that C. cuculus is the second most landed gurnard species on the Portuguese coast (Rocha et al. 2018), justifying the higher fishing mortality and exploitation rate estimated in the present study.

Data on age and growth are essential for the understanding of vital traits of species and populations (Pontual et al. 2002), they are the most frequently used life-history parameters to identify putative stocks (Begg 2005) and, together with mortality, they are important for understanding population dynamics (Ralston and Williams 1988). Although commercially important in European waters, gurnards are mainly by-catch, often discarded, and reported under “mixed gurnards”, with incomplete biological and fishing exploitation information by species. The results of this study will contribute to knowledge of the biological parameters of T. lyra and C. cuculus, helping to define stock characteristics and future management strategies for sustainable exploitation, as recommended by the ICES (ICES 2006). However, additional information on the reproductive strategy of these and other gurnard species and their mixed landings should be obtained for better decision-making in fisheries management.