Estimating biomass, fishing mortality, and &quot;total allowable discards&quot; for surveyed non-target fish
Reid, D. G.
Gerritsen, H. D.
Farnsworth, K. D.
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Shephard, S. Reid, D. G.; Gerritsen, H. D.; Farnsworth, K. D. (2014). Estimating biomass, fishing mortality, and &quot;total allowable discards&quot; for surveyed non-target fish. ICES Journal of Marine Science 72 (2), 458-466
Demersal fisheries targeting a few high-value species often catch and discard other &quot;non-target&quot; species. It is difficult to quantify the impact of this incidental mortality when population biomass of a non-target species is unknown. We calculate biomass for 14 demersal fish species in ICES Area VIIg (Celtic Sea) by applying species-and length-based catchability corrections to catch records from the Irish Groundfish Survey (IGFS). We then combine these biomass estimates with records of commercial discards (and landings for marketable non-target species) to calculate annual harvesting rates (HR) for each study species. Uncertainty is incorporated into estimates of both biomass andHR. Our survey-based HR estimates for cod and whiting compared well with HR-converted fishing mortality (F) estimates from analytical assessments for these two stocks. Of the non-target species tested, red gurnard (Chelidonichthys cuculus) recorded some annual HRs greater than those for cod or whiting; challenging &quot;Pope's postulate&quot; that F on non-target stocks in an assemblage will not exceed that on target stocks. We relate HR for each species to two corresponding maximum sustainable yield (MSY) reference levels; six non-target species (including three ray species) show annual HRs &gt;= HRMSY. This result suggests that it may not be possible to conserve vulnerable non-target species when F is coupled to that of target species. Based on biomass, HR, and HRMSY, we estimate &quot;total allowable catch&quot; for each non-target species.