Abstract
Intense fishing pressure and climate change are major threats to fish population and 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 recent (2019) and historical (1971-1982) life-history parameters and overwintering HSI to analyze the fishing pressure and climate change effects on the overall population and overwintering phase of L. crocea . The length-based analysis indicated serious overfishing of L. crocea , characterized by reduced catch, size truncation, constrained distribution, and advanced maturation causing a recruitment bottleneck. The overwintering HSI modeling results indicated that climate change has led to decreased sea surface temperature during L. croceaoverwintering 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. croceasubsequently 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 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 of L. 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 have caused overfishing and declined catches in Canada, Iceland, and China (Pauly et al. 2011, Du et al. 2014, Liang and Pauly 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 adult survival), and habitat suitability can cause overfishing of exploited species. In this context, recruitment bottleneck and habitat bottleneck are most well documented (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 by removing large fish exacerbated by size-selective gear. 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 and mega-spawner can weaken the reproductive potential of a fish stock, called ‘recruitment bottleneck’ (Doherty et al. 2004). Such bottlenecks are visible in 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).
Climate change-caused environmental conditions shift can have negative effects on fish population (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 lose several fish’s climatically suitable habitat, for examples, Norwegian herring, Maine cod and Mid-Atlantic Bight winter flounder (Bell et al. 2015, Pershing et al. 2015).
Heavily fishing activities and shift in environmental conditions can have combined effects on fishery collapse, especially for long-lived species (Rose 2004, Hsieh et al. 2009, Gascuel et al. 2014). 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 to 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, no example exists 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) (Szuwalski et al. 2017). To fill the knowledge gaps, we require a species that: first, under intensive fishing pressure; second, has specific habitat requirements; third, the habitat of which is affected by rapid climate-induced habitat suitability variation; fourth, has been reliably assessed over a long period by field surveys.
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: first, L. crocea ranked top among the four major marine economical fishes in China in last century (Zhang et al. 2010) but suffered 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).Second, 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). Third, L. crocea is a migratory fish which conduct climatic migrations (e.g. movements driven by physiological tolerances of individuals to environmental factors such as temperature or salinity) and gametic migrations (e.g. movements that increase reproductive success of individuals by promoting gonad development, increasing sexual encounter rates, or increasing the survival of offspring) between offshore water and coastal water during autumn-winter and spring-summer respectively (Fig. 1A).