Abstract
Intense fishing pressure and climate change are major threats to coastal fisheries. Larimichthys crocea (large yellow croaker) is a long-lived fish, which performs seasonal migrations from its spawning and nursery grounds along the coast of the East China Sea (ECS) to overwintering grounds offshore. This study used length-based analysis and habitat suitability index (HSI) model to evaluate current life-history parameters and overwintering habitat suitability ofL. crocea , respectively. We compared both life-history parameters and overwintering HSI between recent (2019) and historical (between 1971 to 1982) to analyze the fishing pressure and climate change effects on the overall population and overwintering phase of L. crocea . In the context of overfishing, the length-based analysis indicated serious overfishing of L. crocea , characterized by reduced catch yield, size truncation, constrained distribution, and advanced maturation in the ECS, namely recruitment bottleneck. In the context of climate change, the overwintering HSI modeling results indicated that climate change has led to decreased sea surface temperature during L. crocea overwintering phase over the last half-century, which in turn led to area decrease and an offshore-oriented shifting of optimal overwintering habitat of L. crocea . The fishing-caused size truncation may have constrained the migratory ability and distribution of L. crocea subsequently led to the mismatch of the optimal overwintering habitat against climate change background, namely habitat bottleneck. Hence, while heavily fishing was the major cause of L. crocea fishery collapse, climate-induced overwintering habitat suitability may have intensified the fishery collapse of L. crocea population. It is important for management to take both overfishing and climate change issues into consideration when developing stock enhancement activities and policy regulations, particularly for migratory long-lived fish that share a similar life history to L. crocea . Combined with China’s current restocking and stock enhancement initiatives, we propose recommendations for future restocking ofL. crocea in China.
Key words: Larimichthys crocea, overfishing, climate change, length-based analysis, HSI model, East China Sea.
INTRODUCTION
Globally, heavily fishing activities and climate change are rapidly reducing the abundance of many marine organisms and increasing the likelihood of species extinction (Hoegh-Guldberg and Bruno 2010, Cinner et al. 2012, Burgess et al. 2013, Payne et al. 2016). For instances, intensive fishing and climate change had caused overfishing and declined catches in Canada, Iceland, and China (Pauly et al. 2011, Du et al. 2014, Liang and Pauly 2017). In addition, ‘fishing-induced life-history variation’ at population level, together with ‘fishing down the marine food web’ and/or ‘trophic cascade’ at ecosystem level have been observed and demonstrated in variety contexts through theoretical and empirical evidence (Essington et al. 2006, Gascuel et al. 2014, Kuparinen et al. 2016, Szuwalski et al. 2017). Previous studies showed that fishing pressures and climate change can affect the (i) the life-history strategy of individuals, via impacts on physiology, morphology and behavior (Ba et al. 2016, Olafsdottir et al. 2016); (ii) the population dynamics, via changes to key population processes throughout an organism’s life-history and habitat suitability (Perry et al. 2005). Hence, bottlenecks of any life-history stage (e.g. spawning, hatching, larval survival, recruitment settlement, growth, and death), and habitat suitability can cause overfishing of exploited species. In this context, recruitment bottleneck and habitat bottleneck are probably the most widely known bottlenecks (Almany and Webster 2006, Caddy 2011). Correspondingly, the potential cause of overfishing is mismanagement because of a poor understanding of recruitment bottleneck and habitat bottleneck that constrain the productivity of the overall population.
Fishing alters the size structure throughout remove large fish due to the size-selective of gears. Heavily fishing can diminish the ability of fish to reproduce (recruitment overfishing) and/or constrain the overall recruitment ability before they can fully realize their growth potential (growth overfishing) (Diekert 2012) via size truncation effect (STE) (Berkeley et al. 2004, Ottersen et al. 2006, Froese et al. 2008, Langangen et al. 2019). This effect states that population shifts with decreasing body sizes and advancing maturation characteristic of the life-history changes induced by fishing (Berkeley et al. 2004, Anderson et al. 2008, Bell et al. 2015). Hence, fishing for juveniles (before reaching first maturity) and mega-spawner can weaken the reproductive potential of a fish stock, called ‘recruitment bottleneck’ (Doherty et al. 2004). Such bottlenecks are visible in respective long-term time series and are a common cause of collapse in intense fished stocks, for example in Western cod, Pacific rockfish and North Sea ground fish (Harvey et al. 2006, Poulsen et al. 2007, Froese et al. 2008).
The impacts of overfishing do not occur in isolation, but against a background of existing climate change-caused environmental conditions shift (Graham et al. 2011, Johnson et al. 2011). In general, species’ distribution patterns are relative with both life-history strategies (Anderson et al. 2013) and physiology tolerance on environmental variables, such as sea surface temperature (SST), chlorophyll-a concentration (Chl-a), sea surface salinity (SSS), currents et al. (Guan et al. 2013, Yu and Chen 2018). Environmental shift can selectively affect the habitat suitability of target species (Farrell et al. 2008). Lower habitat suitability of any life-history stage can lead to species-specific ‘habitat bottleneck’ and latter can have large consequences for climate-induced fishery phenomena, for examples, Norwegian herring, Maine cod and Mid-Atlantic Bight winter flounder (Bell et al. 2015, Pershing et al. 2015).
Heavily fishing activities and environmental conditions shift can have combined effects on fishery collapse, especially for long-lived species (Rose 2004, Hsieh et al. 2009). Specifically, some studies suggested that long-lived species are expected to have slower demographic response to climate change (Berteaux et al. 2004, Wilson et al. 2010). Additionally, fishing-caused STE can exacerbate long-lived fish degradation via diminishing ‘bet-hedging’ capacity, including the ability of migrate and avoid poor areas, having flexibility in spawning times and locations, and production of high-quality offspring that survive in a broader suite of environmental conditions, for adapting to rapid climate change (Bell et al. 2015). However, there is no relevant examples exist that demonstrate the STE and the climate-induced effects on long-lived migratory fish in the most heavily fishing (and minimally managed) marine ecosystem in the world: the East China Sea (ECS). To fill the knowledge gaps, we require a species that (i) under intensive fishing pressure; (ii) has specific habitat requirements; (iii) the habitat of which is affected by rapid climate-induced habitat suitability variation; (iv) can be reliably counted by a long-term field survey.
In the following, we provide an appropriate example by discussing changes in specific population dynamic of an overexploited, long-lived, migratory fish in the ECS, the large yellow croaker (Larimichthys crocea ). The collapse of L. crocea represents an interesting example to explore both heavily fishing and climate change on overall population: (i) L. crocea ranked top among the four major marine economical fishes in China in last century (Zhang et al. 2010) but encountered collapse since the 1980s. The latest International Union for Conservation of Nature (IUCN) Red List of Threatened Species labelled L.croce s as ‘critically endangered (CR)’ (Liu et al. 2020); (ii) L. crocea is a long-lived species with maximum age 21 years in 1960s (Zhang et al. 2017). Accompanied by population collapse, the L. crocea population in the ECS is characterized by decreased maximum age and body size, and advanced maturation (Ye et al. 2012). (iii) L. crocea is a migratory fish which conduct climatic migrations and gametic migrations between offshore water and coastal water during autumn-winter and spring-summer respectively (Fig. 1A).