Abstract
AbstractSynthesis studies of fish stocks worldwide suggest improving status of mainly target species that are fully assessed. Other analyses, primarily based on catch data alone, but which include a wider range of species as well as bycatch, present a different view. Catch‐only analyses could be more robust if fishery‐independent data were used and discards accounted for. We develop a model that uses only survey biomass at length and landings data to estimate fishing mortality, spawning stock biomass (SSB) and discards. An analysis of species from the North Sea shows the model results compare well with most fully assessed stocks. When applied to bycatch species with limited data, trends in fishing mortality and SSB typically reflect those of the target species. In the last decade, mean fishing mortality rates have tended to decline, while mean SSB has increased. Despite increasing SSB, recent mean recruitment appears to have been lower than previously which may limit future biomass recovery. Species usually associated with more northerly distributions appear to show the greatest effect of weaker recruitment, which may be linked to climate. Estimated discards have tended to decline in magnitude as a result of reduced fishing mortality and associated lower total catches. The model offers a simple way to use both landings and survey data to obtain more detailed population trends for data limited species.
Highlights
Some recent studies have documented improving status of many fish stocks worldwide (Cardinale et al, 2013; Fernandes & Cook, 2013; Worm et al, 2009)
Costello et al (2012), for example, suggest that the status of unassessed species may be worse than that of assessed species, while Piet, van Hal, and Greenstreet (2009) suggest that the fishing mortality on bycatch species may be higher than target species in the North Sea
We extended the model to account for discards and refer to it as the Landings- Survey-Discard (LSD) model
Summary
Some recent studies have documented improving status of many fish stocks worldwide (Cardinale et al, 2013; Fernandes & Cook, 2013; Worm et al, 2009). While a fully length-based assessment method such as catch-at-size analysis (Sullivan, Lai, & Gallucci, 1990) or a method with an age-structured model that uses length observations such as Stock Synthesis (Methot & Wetzel, 2013) might be applicable, these generally work best with well-sampled length frequency distributions to estimate model parameters These are not always available for species of low abundance. We develop a method of assessment based on the Collie–Sissenwine approach (Collie & Sissenwine, 1983) that is only weakly dependent on the survey length data to determine discards given the landings (Heath & Cook, 2015) This approach uses a population dynamics model cast purely in terms of numbers, avoiding the need to estimate age or growth rates. The studies provide a snapshot of biomass and exploitation rate but as adequate survey data exist at least since the early 1980s, it is possible to reconstruct trends over three decades using the new model and compare these with those obtained by complex assessment models
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