Threshold-dependent climate effects and high mortality limit recruitment and recovery of the Kattegat cod

Abstract

Cod in the Kattegat is one of the most dramatic examples of stock collapse, where despite large management efforts, almost no signs of recovery have been observed. We investigate how multiple physical and biological factors could potentially influence recruitment and recovery of Kattegat cod, using non-additive threshold models. In contrast to previous studies on recruitment dynamics of Kattegat cod Gadus morhua, we found that recruitment variability may be explained by a combination of the size of the spawning stock and external conditions (i.e. sea surface temperature and oxygen concentrations), but only during periods of low stock size. Our results indicate that the long-term decrease and the present poor state of the Kattegat cod stock is likely caused by high total mortality rates and stock-size dependent effects of climate which together are currently preventing recovery. In addition, we illustrate how only a drastic reduction in total mortalities, primarily by limiting unintended bycatch and discards, may promote a recovery of the stock. This knowledge is important for evaluating the success or failure of various management measures which have been employed to recover the stock and for developing future management strategies which can take the environmental and/or ecosystem impacts into account.KEY WORDS: Recovery · Climate · Thresholds · Recruitment · Cod · Management · Kattegat · ModellingResale or republication not permitted without written consent of the publisher

Impacts of the local environment on recruitment: a comparative study of North Sea and Baltic Sea fish stocks

“…In contrast, among the stocks where the environment explained a large part of the survival variability, the biomass was lower, unknown, or fluctuating around MSY(B trigger ) (Table 2, deviance explained .80%). These observations are in agreement with the higher environment-recruitment link when the fishing pressure has eroded the age and length composition of the spawners (Longhurst, 2002;Ottersen et al, 2006) and when the biomass is relatively low (Brander, 2005;Lindegren and Eero, 2013). Different environmental conditions, described by different environmental regimes, can provoke a change in the productivity of the stock and hence modify the relationship between the spawningstock biomass and the recruitment (Köster et al, 2009).…”

“…Planque and Frédou, 1999;Mackenzie, 2000;Olsen et al, 2011). Other environmental variables have been less frequently studied but have nonetheless shown a significant impact on recruitment, such as oxygen (Köster et al, 2005b;Lindegren and Eero, 2013), salinity (Heikinheimo, 2008), and eutrophication (Pihl et al, 2005).…”

Relationship between recruitment of Japanese sardine (Sardinops melanostictus) and environment of larval habitat in the low‐stock period (1995–2010)

“…For example, if the spawning ecology totally changed in 1994, SSE will be minimized with threshold SSB ranged from 487,000 (SSB in 1993) from 569,000 (SSB in 1994). A similar detecting method is usually used in studies on fish of which long-term stock trend can be explained by a threshold-dependent process (Ciannelli, Chan, Bailey, & Stenseth, 2004;Lindegren & Eero, 2013;Llope et al, 2011).…”

Biomass limit reference points are sensitive to estimation method, time‐series length and stock development

“…BT-slopes were derived using the following four methods: P0.5 (a), P0.8 (b), HS (c) and 0.2B max (d). The grey scale represents the uncertainty of the S-slope (dark grey is high uncertainty) & Lindegren & Eero, 2013). Such high variability is particularly prevalent in short-lived, fast-growing and early maturing species of small pelagic fish (e.g.…”