The composition and structure of fish assemblages caught by trawl in the Levantine Sea (eastern Mediterranean) were summarized using a fishing-sensitive univariate indicator. This metric, called the "Large Fish Index" (LFI), has been developed in the North Sea since 2000 and is applied and adapted here for the first time in the eastern Mediterranean. It was defined as the fish biomass fraction above a pre-determined length threshold, expressed as proportion of the total fish biomass. Trawl-caught assemblages in the warm, oligotrophic, highly invaded Levant were found to be characterized by small fishes. "Large fish" were thus delineated as 20-30 cm (total length) for this region, as opposed to 40 cm set in the North Sea. Desired minimum LFI proportions were set as 0.5 for fishes longer than 20 cm and 0.3 for fishes longer than 30 cm and the actual values found in Israeli bottom trawl surveys from 2008 to 2012 were 0.3 and 0.155 respectively. These low LFI values for the small fishes caught by trawlers attest to the ecological unsustainability and economic inefficiency of bottom trawling in Israel and provide a tool for managers with which to assess the state of the fishery and the general health of the ecosystem.
La composición y estructura de las comunidades de peces capturados por arrastre en el mar de Levante (Mediterráneo Oriental) se expresó mediante un indicador univariado sensible a la pesca. Esta medida, denominada "Índice de grandes peces" (IGP), se desarrolló en el Mar del Norte desde 2000 y se aplica y se adapta por primera vez al Mediterráneo oriental. Se define como la fracción de biomasa de peces por encima de un umbral de longitud predeterminada, expresada como proporción de la biomasa total. Se encontró que los conjuntos de peces capturados en el mar de Levante, cálido, oligotrófico, y con muchas especies invasoras se caracterizan por estar formados de peces pequeños. Se definieron los "peces grandes" para esta región aquellos superiores a 20 o 30 cm de longitud total, en lugar de los 40 cm establecidos en el Mar del Norte. Las proporciones mínimas deseables de IGP se establecen en 0.5 para peces de más de 20 cm y 0.3 para los peces de más de 30 cm mientras que los valores reales que se encuentran en las campañas de arrastre de fondo israelíes 2008-2012 fueron 0.3 y 0.155, respectivamente. Estos bajos valores señalan la insostenibilidad ecológica y la ineficiencia económica de la pesca de arrastre en Israel y proporcionan una herramienta para los gestores con los que evaluar el estado de la pesca y la salud general del ecosistema.
Addressing anthropogenically induced changes in fish communities through community-level or ecosystem-level indicators is imperative to the proper management of fisheries (
The Levantine Sea is a highly oligotrophic body of water, with high salinity and temperatures (e.g.
A sustainable fishery is expected to extract large animals, which have had time to reproduce and contribute to the gene pool (
Data were collected from 228 commercial trawl hauls using two-panel nets with sweeps (54-72 m between trawl doors) and a 40 mm PE diamond mesh at the codend. As these codends are considered highly non-selective and allow few fishes to escape (
The length-weight relationships of fishes were used to allocate biomass to each measured specimen in samples. The LFI was then calculated sensu
where
Note that in the numerator for this equation, the summation is carried out across l>40 cm only (i.e. the North Sea LFI value) while here we proceeded to calculate it for the entire size spectrum.
Mediterranean trawl fisheries target much smaller finfish than Atlantic European fleets (
Our 2008-2012 surveys yielded 72812 bony fish length measurements, out of which 70461 (96.7%) were below 20 cm, and 72150 (99%) were below 30 cm. In the North Sea,
As Mediterranean trawlers rely more on juvenile fishes than Atlantic ones (
We also calculated the LFI separately for each year (
The ‘smallness’ of Levantine fish assemblages may also be rooted in invasive species. Of the 72812 fishes measured in this study, 56307 (58.4%) were Lessepsian Migrants—Nonindigenous fishes of Indo-Pacific origin. The three most common species in this study, accounting together for 30% of the specimens, were nonindigenous: 1) The tiny (typically 5-10 cm total length) pony fish
In sizes of 12.5-18 cm, indigenous species slightly outnumber nonindigenous ones, but many indigenous species are also by-caught in small sizes of 5-10 cm total length (
In
Ex-fishery factors contributing to small sizes in Levantine fishes
The faunistic invasion via the Suez Canal has profoundly and irreversibly altered the composition of the Levantine ichthyofauna (
One main natural cause in explaining overall diminished sizes in Levantine fishes is undoubtedly Bergmann’s rule, stating that within a broadly distributed species, specimens from cooler climates tend to be larger than conspecifics from warmer climates (
The 0.3 LFI level was proposed for the North Sea (using the 40 cm length) to denote 1983 levels. The actual values declined to a low of 0.05 in 2001, and recovered to 0.22 in 2008 (
The LFI was created to serve as an indicator of the “general health” of the demersal fish community. Its strengths lie in its simplicity, wide acceptance and sensitivity to fishing pressure (
We do not yet know how long it would take the Levantine fishery to respond to anomalous year classes, recruitment events or even fishery management measures, as the general state of knowledge of Levantine fisheries lags considerably behind that of the North Sea. Without this knowledge we forewarn against jumping to distracting and possibly misleading conclusions due to exceedingly high or low single-year values. This must also be taken into consideration during survey design, so that variables affecting size spectra are equally sampled. For example, in our study the low values in 2008 and high values in 2012 (
There are several obvious drawbacks of the LFI in its current form. First, invertebrates and especially shrimps and squids, which comprise a significant part of the catch of Israeli bottom trawlers, are excluded. Second, The LFI30 is insensitive to changes in the size classes of some of the commonest commercial species such as goatfishes or the smaller sparids, which have an Lmax smaller than 30 cm. The LFI20, however, makes the index more prone to recruitment class strength. Third, because samples rather than the entire catch were used the impact of larger and heavier specimens, caught much more sporadically than smaller-bodied species, was considerable. For example, the omission of sharks and guitarfish from the analysis decreased LFI values by 0.05-0.1, underlining the importance of proper species complex allocation. Finally, the inclusion of both by-caught species and target species in the catch means that large by-caught species regularly exceeding 30 cm (like the poisonous puffer
Despite these shortcomings, we maintain that the proposed LFI indicator can serve particularly well for several purposes. First, fishery managers, scientists and the public can be alerted to the unsustainable nature of trawling in the Levant today and the small size of the exploited assemblage with a simple to understand index. Second, the seasonal evolution of the LFI also allows makes it possible to detect when the fishery produces the largest juvenile proportion in the catch. The wastage incurred by fishing in the summer is well reflected in
The LFI can be applied in the Mediterranean and is presented here as a useful tool for examining the sustainability of bottom trawling in Israel. Even taking into consideration the smallness of Levantine ichthyofauna, the environmental performance of Israeli trawlers still seems rather poor, with fishes larger than 30 cm only accounting for 15% of the fish biomass. We therefore encourage managers to use measures such as seasonal closures, MPAs or gear changes in order to increase trawl selectivity. Also, we suggest that the arbitrary values set here as characterizing “Good Environmental Status” are revisited and discussed further if and when such measures are used. As advised for northern European seas (
This study was supported by a research grant from the Israeli Ministry of Environmental Defense. The authors also wish to thank Dr. Jason Goldstein for the useful suggestions which have led to significant improvements of this manuscript.
Species | a | b | SE(b) | R2 | n | Min | Max |
---|---|---|---|---|---|---|---|
|
0.0398 | 2.6263 | 0.027399 | 0.9949 | 3 | 7.5 | 33.5 |
|
0.011 | 2.972 | 0.007174 | 0.9899 | 43 | 3.5 | 23 |
|
0.0161 | 2.9869 | 0.005903 | 0.9659 | 30 | 6.6 | 10.5 |
|
0.017 | 3.1274 | 0.015471 | 0.9577 | 21 | 4 | 9.5 |
|
0.0007 | 3.8389 | 0.011064 | 0.9776 | 10 | 9.4 | 13.5 |
|
0.0019 | 2.894 | 0.009562 | 0.9059 | 34 | 10.5 | 25 |
|
0.0087 | 3.0721 | 0.011529 | 0.9022 | 68 | 4.5 | 11.2 |
|
0.0125 | 2.8666 | 0.03419 | 0.77 | 11 | 8 | 11.9 |
|
0.0756 | 2.546 | 0.00942 | 0.981 | 33 | 15 | 66 |
|
0.011 | 2.9293 | 0.009304 | 0.9502 | 170 | 5 | 24 |
|
0.0106 | 3.0636 | 0.008272 | 0.9535 | 92 | 6 | 16.3 |
|
0.0301 | 2.183 | 0.021361 | 0.8173 | 12 | 5.2 | 7.5 |
|
0.0284 | 2.4409 | 0.006075 | 0.8306 | 199 | 6 | 15.5 |
|
0.0255 | 2.8223 | 0.023148 | 0.8649 | 14 | 2 | 10.5 |
|
0.0089 | 3.108 | 0.009288 | 0.9843 | 30 | 11.7 | 54 |
|
0.0065 | 3.0486 | 0.01641 | 0.909 | 78 | 4 | 12.5 |
|
0.0071 | 3.021 | 0.011477 | 0.95 | 92 | 5.5 | 21 |
|
0.0011 | 3.0663 | 0.006989 | 0.9731 | 59 | 17 | 83 |
|
0.0167 | 2.7044 | 0.007824 | 0.9683 | 37 | 6 | 16 |
|
0.0051 | 3.2185 | 0.006917 | 0.9855 | 70 | 6 | 21 |
|
0.011 | 3.1353 | 0.004401 | 0.9522 | 57 | 9 | 14.5 |
|
0.0242 | 2.8583 | 0.012587 | 0.9475 | 9 | 9 | 13.5 |
|
0.014 | 3.023 | 0.009064 | 0.967 | 12 | 18 | 35 |
|
0.0257 | 2.8483 | 0.007489 | 0.9448 | 11 | 15.5 | 22 |
|
0.0171 | 3.0011 | 0.004452 | 0.9816 | 30 | 14 | 25.5 |
|
0.0139 | 3.0335 | 0.004616 | 0.9508 | 32 | 12.5 | 24 |
|
0.0053 | 3.1562 | 0.006841 | 0.976 | 37 | 8.2 | 14.5 |
|
0.0001 | 3.4528 | 0.006842 | 0.9555 | 36 | 34.4 | 74.5 |
|
0.0006 | 3.417 | 0.012928 | 0.9921 | 9 | 17.5 | 63.5 |
|
0.0081 | 2.9224 | 0.006573 | 0.9764 | 38 | 6 | 15 |
|
0.013 | 2.958 | 0.006521 | 0.996 | 34 | 8.5 | 85 |
|
0.0086 | 3.1726 | 0.009908 | 0.9252 | 134 | 3.5 | 10.1 |
|
0.0088 | 2.9832 | 0.012074 | 0.9731 | 16 | 8 | 24 |
|
0.008 | 3.074 | 0.012671 | 0.993 | 10 | 31.5 | 97 |
|
0.0001 | 3.3501 | 0.011223 | 0.983 | 40 | 13 | 99.5 |
|
0.0073 | 3.1241 | 0.023896 | 0.9704 | 7 | 5.5 | 12.5 |
|
0.0166 | 2.9725 | 0.005856 | 0.9896 | 52 | 6 | 16 |
|
0.014 | 3.1609 | 0.009218 | 0.9403 | 42 | 5.5 | 9.5 |
|
0.0161 | 2.9546 | 0.006961 | 0.9829 | 145 | 3.4 | 14.5 |
|
0.0225 | 2.8227 | 0.00821 | 0.9907 | 57 | 5 | 56.5 |
|
0.0243 | 2.8686 | 0.005618 | 0.9906 | 103 | 5.5 | 42 |
|
0.012 | 2.999 | 0.003894 | 0.9684 | 128 | 6 | 19.5 |
|
0.00005 | 3.7074 | 0.026623 | 0.9966 | 6 | 25 | 51.5 |
|
0.0057 | 3.3492 | 0.009555 | 0.9412 | 121 | 4.5 | 12.5 |
|
0.0137 | 2.9418 | 0.012391 | 0.9504 | 6 | 9 | 11.5 |
|
0.0121 | 3.0175 | 0.004074 | 0.9852 | 86 | 7 | 25 |
|
0.0049 | 3.264 | 0.017153 | 0.9908 | 5 | 21 | 61 |
|
0.008 | 2.8543 | 0.011771 | 0.9501 | 57 | 4 | 12 |
|
0.0035 | 3.233 | 0.005637 | 0.989 | 74 | 7.5 | 37 |
|
0.0792 | 2.312 | 0.000785 | 0.8952 | 12 | 7 | 13 |
|
0.0107 | 3.0012 | 0.007972 | 0.9686 | 182 | 5 | 22 |
|
0.0054 | 3.2824 | 0.003276 | 0.9827 | 167 | 7.5 | 23 |
|
0.0101 | 3.0769 | 0.007346 | 0.9702 | 168 | 4 | 23.5 |
|
0.0247 | 2.781 | 0.005207 | 0.996 | 7 | 8.5 | 21 |
|
0.0036 | 3.1103 | 0.005503 | 0.984 | 45 | 7.5 | 22 |
|
0.0206 | 2.5365 | 0.012296 | 0.8528 | 26 | 11 | 22 |
|
0.0055 | 3.3254 | 0.003376 | 0.9753 | 311 | 5 | 16.5 |
|
0.0188 | 2.9314 | 0.009847 | 0.9679 | 18 | 7 | 14.5 |
|
0.0127 | 3.0257 | 0.005696 | 0.9873 | 143 | 4 | 24 |
|
0.033 | 2.7504 | 0.008504 | 0.9935 | 37 | 6.5 | 44 |
|
0.0034 | 3.2537 | 0.00794 | 0.9669 | 161 | 5 | 23 |
|
0.0095 | 3.1274 | 0.004264 | 0.9929 | 19 | 11 | 22.5 |
|
0.0107 | 3.0889 | 0.01148 | 0.9485 | 10 | 9 | 14.5 |
|
0.015 | 2.929 | 0.007997 | 0.9782 | 14 | 6 | 10 |
|
0.0039 | 3.2882 | 0.003769 | 0.9672 | 261 | 6 | 21 |
|
0.0025 | 2.6422 | 0.003634 | 0.9306 | 95 | 9 | 24 |
|
0.0112 | 2.9211 | 0.054406 | 0.9882 | 15 | 13.5 | 25.5 |
|
0.0214 | 2.938 | 0.005881 | 0.9635 | 35 | 11 | 23.5 |
|
0.0039 | 3.1538 | 0.006612 | 0.9918 | 84 | 6 | 36 |
|
0.0034 | 3.291 | 0.004591 | 0.9845 | 130 | 6.5 | 30.5 |
|
0.0113 | 2.8494 | 0.00593 | 0.9807 | 17 | 21.5 | 51.5 |
|
0.0146 | 3.0657 | 0.004879 | 0.999 | 8 | 7 | 24 |
|
0.0258 | 2.855 | 0.014324 | 0.9737 | 12 | 6.5 | 12 |
|
0.0097 | 2.8389 | 0.013651 | 0.9918 | 5 | 10 | 21.5 |
|
0.013 | 3.128 | 0.006939 | 0.9983 | 8 | 7 | 19.5 |
|
0.0085 | 3.1225 | 0.006135 | 0.9726 | 23 | 21 | 39.5 |
|
0.0095 | 3.0729 | 0.009257 | 0.9845 | 36 | 6.3 | 22 |
|
0.0118 | 3.159 | 0.007568 | 0.9656 | 97 | 4.5 | 10 |
|
0.0145 | 2.8942 | 0.014034 | 0.9526 | 36 | 8.5 | 23.5 |
|
0.0063 | 3.0762 | 0.004672 | 0.9813 | 34 | 8.5 | 18.5 |
|
0.0721 | 2.3086 | 0.052164 | 0.9068 | 4 | 14.5 | 17.5 |
|
0.0866 | 2.5455 | 0.018968 | 0.9412 | 9 | 12 | 26 |
|
0.0089 | 2.8528 | 0.004931 | 0.9384 | 92 | 9.5 | 32 |
|
0.004 | 3.0077 | 0.009013 | 0.985 | 36 | 9.5 | 40 |
|
0.0103 | 3.0631 | 0.004677 | 0.9788 | 43 | 7.5 | 17.2 |
|
0.0116 | 2.9541 | 0.004478 | 0.8523 | 209 | 8 | 17 |
|
0.0149 | 2.8043 | 0.004775 | 0.9387 | 232 | 5 | 15 |
|
0.0164 | 3.0335 | 0.010481 | 0.9796 | 64 | 4 | 20.5 |
|
0.0112 | 2.8802 | 0.007566 | 0.9618 | 34 | 10 | 24 |
|
0.104 | 2.2155 | 0.022454 | 0.932 | 10 | 5.5 | 10.5 |
|
0.0018 | 3.4875 | 0.007315 | 0.9877 | 15 | 14 | 26 |
|
0.009 | 2.9372 | 0.004468 | 0.9798 | 205 | 6.5 | 12 |
|
0.0088 | 3.0041 | 0.010208 | 0.9808 | 47 | 7 | 24.5 |
|
0.0109 | 2.8978 | 0.004754 | 0.9885 | 103 | 5.5 | 29.5 |
|
0.0029 | 2.5504 | 0.01876 | 0.895 | 20 | 16 | 44 |
|
0.0115 | 3.0112 | 0.006095 | 0.9834 | 45 | 8 | 23 |
|
0.0085 | 3.1033 | 0.004528 | 0.9538 | 162 | 6 | 20.5 |
|
0.005 | 3.3081 | 0.006462 | 0.9773 | 68 | 7 | 17.5 |
|
0.0123 | 3.1094 | 0.007136 | 0.9826 | 29 | 9.5 | 23.5 |
|
0.0034 | 3.4817 | 0.014933 | 0.9656 | 7 | 11.5 | 19 |
|
0.0029 | 2.7431 | 0.020001 | 0.9905 | 21 | 4.5 | 47 |