Evidence of difference in landings and discards patterns in the English Channel and North Sea Rajidae complex fishery
Introduction
Worldwide, 34% of assessed fish stocks are overfished (FAO, 2020, Hilborn et al. 2020). In response, the demand for adequate management measures has been growing. Currently, quotas in a variety of forms (species specific, complex, multispecific) are widely used to regulate fishing pressure (Karagiannakos 1996, Marchal et al. 2016, Newell et al. 2005), with increased requirements for stock assessment models (Carruthers et al., 2014).
Since analytical stock assessments have primarily been implemented for commercially valuable species, until recently, little attention has been paid to bycatch species such as Rajidae, resulting in a relatively low data collection effort (Stevens et al., 2000). However, Rajidae are particularly sensitive to fishing, due to their specific life history traits (Dulvy et al., 2000). Most of these species are characterized by long life expectancy and low fecundity, which reduces their resilience to fisheries (Dulvy et al., 2014, Dulvy & Reynolds, 2002). Furthermore, being top predators, Rajidae play an active role in the ecosystem’s top down regulation. All of these factors underline the fact that a decline of Rajidae is considered as a potential threat to the whole ecosystem (Stevens et al., 2000).
In the English Channel and North Sea (ECNS) area, 3a, 4 and 7d ICES (International Council for the Exploration of the Sea) statistical rectangles, Rajidae are mainly taken as a bycatch in flatfish fisheries. Most of these by-catches are taken by trawlers and to a lesser extents gillnetters, from France, the United Kingdom, Netherlands and Belgium. The mandatory reporting of landings by species for Rajidae in the ECNS only started in 2009. Before 2009, all Rajidae landings were binned into a single category. Although misidentification might occurred the reports from 2009 to 2018 are supposed to be consistent with no clear improvement or deterioration of the quality of identification. In addition, the absence of otoliths and the complexity of vertebrae age-reading has hindered the provision of age-structured proxies for relative abundance. Hence, with little data available to support analytical stock assessments, Rajidae have been considered Data Limited Stocks (DLS) and assessed with trend-based analysis methods by the ICES (ICES, 2019a).
Worldwide, about 80% of fish stocks are not analytically assessed, or not assessed at all, principally due to data limitations (Costello et al., 2012). Consequently, models have been developed to assist stock assessments in data-limited situations, with a majority of them based on surplus-production models, such as the Schaefer (1954) or Pella Tomlinson (1969) models (Pedersen & Berg 2017, Martell & Froese, 2013, Wetzel & Punt, 2015). Surplus-production models require less information compared to age- or size-structured stock assessment models. Still, reliable time series of catch data and one or more biomass indices are usually needed. The catch data are used to estimate the removals from the stock; those fish that die from fishing. This type of model provides proxies of reference points such as the biomass (BMSY) or the fishing mortality (FMSY) at the maximum sustainable yield (MSY) that are currently used to provide stock management advices. However, they might have high sensitivity to inputs (Geng et al., 2020) and not explicitly take into account age structure and the existing delays for elasmobranch between reproduction and recruitment (Musick and Bonfil, 2005). Surplus production models have already been applied to assess several sharks and rays species, in the USA and Australia (Musick and Bonfil, 2005, Cortés et al., 2002) and might be used to assess some of the ICES data limited stocks.
The landings data of Rajidae underestimate the total catches, and thus the removals. They are mainly taken as bycatch species, their TAC is usually reached before the main target species, sole (Solea solea) and plaice (Pleuronectes platessa) TACs, and a substantial fraction of Rajidae are discarded. In the case of Rajidae stocks in the ECNS, discards data come from observer programs that were first dedicated to flatfish, in the area (ICES, 2003). For that reason, Rajidae discards data are sparse.
Discarding in demersal fisheries in the ECNS occurs for several reasons. First fish may be smaller than the minimum size restrictions set within the fishery (Feekings et al., 2012). In 2020, the Rajidae minimum landing size varied between countries with, a total length of 45 cm in France, 50 cm in Belgium and 55 cm in Netherlands and a total width of 40 cm in the United Kingdom. These four countries fleets occur constantly in the area and represent the essential of Rajidae catches. These country specific minimum landing sizes were already implemented at the end of the time serie used. In addition to size restrictions fish may be discarded when quota limit landings of certain species while fishing takes place for other species (Batsleer et al., 2015). In this context, Rajidae are discarded to avoid a choke species effect on flatfish, but all Rajidae species, within the complex, might not be affected in the same way by this behaviour. This creates a mutual dependency between landings and discards that makes it essential to estimate their relation dynamics and causality (Rochet & Trenkel, 2005, James et al., 2016). A deep understanding of these elements might offer key elements to estimate catch only model suitability and understand factors that drove retention patterns.
The objectives of this study, based on the three main ECNS Rajidae species: R. clavata, R. brachyura and R. montagui, were, (1) to describe discards and landings trends among species, (2) to assess, based on a complex state space Bayesian model, the impact of discards trends on species stock status and biomass trajectories, and provide proxy reference points and (3) to identify potential reasons of increase in discard rates for some species while not for others.
Section snippets
Data
The surplus-production model used to estimate stock status for the three species requires biomass indices and catch data. In the ECNS Rajidae complex, these data were not directly available in a usable form.
Survey indices
Table 4 shows the differences in catchability between the BTS, and NS-IBTS and CGFS surveys for skates in the ECNS. For all species and biomass combinations, except for the vulnerable biomass index of the CGFS for R. brachyura, the estimated coefficients are significantly (p value <0.05) smaller than 0, implying that the BTS survey has a higher catchability compared to the NS-IBTS and CGFS surveys. For each combination of survey and skate species, the difference in catchability of the NS-IBTS
Landings, discards and biomass dynamics
Data analysis coupled with model outputs, suggests an overfishing event of Rajidae stocks preceding 1990 first management measures. Stocks stayed depleted until the implementation of 2009 management measures that encompass the whole ECNS area. Rajidae stock declines during the 20th century period in the North East Atlantic has already been reported in surrounding fishing areas (Dulvy et al., 2000, Marandel et al., 2019). Stock biomass trends indicated, for all species of the complex, a common
Conclusion
ECNS Rajidae fishery presents clear retention patterns, but further studies are needed to understand which biological, economical and sociological processes are behind these patterns. However, within the complex, retention preferences appeared to affect the stocks’ rebuilding dynamics.
Retention patterns differences in bycatch DLS implies constraints in stock assessment models. These constraints impact catch data reliability and model choices. In the case of complex management, a multispecies
Funding
All financial support was provided by EU Interreg 2 Seas project 2S03-024 SUMARiS.
CRediT authorship contribution statement
Morgane Amelot: Conceptualization, Methodology, Formal analysis, Writing - original draft. Jurgen Batsleer: Conceptualization, Validation, Writing - review & editing, Data curation. Eric Foucher: Conceptualization, Validation, Writing - review & editing, Supervision. Raphaël Girardin: Conceptualization, Validation, Writing - review & editing. Paul Marchal: Conceptualization, Validation, Writing - review & editing, Supervision. Jan Jaap Poos: Conceptualization, Validation, Writing - review &
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
We thank Pascal Lorance for giving us access to Floriane’s Marandel multispecies model code. Noémie Van Bogaert for giving us access to SUMARiS on survival rates preliminary results. Carine Sauger and Catie Mitchell for their help on grammar and spelling and Laurent Dubroca for his help on data handling. We thank the members of ICES WGEF and ICES WGMIXFISH for providing the information needed on catches and fishing effort. Finally, we thank 2 anonymous reviewers, whose comments helped improve
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