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Estimating biomass, fishing mortality, and "total allowable discards" for surveyed non-target fish
Shephard, S.; Reid, D.G.; Gerritsen, H.D.; Farnsworth, K.D. (2015). Estimating biomass, fishing mortality, and "total allowable discards" for surveyed non-target fish. ICES J. Mar. Sci./J. Cons. int. Explor. Mer 72(2): 458-466. hdl.handle.net/10.1093/icesjms/fsu146
In: ICES Journal of Marine Science. Academic Press: London. ISSN 1054-3139, more
Peer reviewed article  

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Keywords
    By catch; Catchability; Stock assessment; Elasmobranchii [WoRMS]; Marine
Author keywords
    Data-poor stocks; Elasmobranchs; Pope's postulate

Authors  Top 
  • Shephard, S.
  • Reid, D.G.
  • Gerritsen, H.D.
  • Farnsworth, K.D.

Abstract
    Demersal fisheries targeting a few high-value species often catch and discard other “non-target” 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 and HR. 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 “Pope's postulate” 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 = 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 “total allowable catch” for each non-target species.

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