Does coastal lagoon habitat quality affect fish growth rate and their recruitment? Insights from fishing and acoustic surveys

Abstract

Ensuring the sustainability of fish resources necessitates understanding their interaction with coastal habitats, which is becoming ever more challenging in the context of ever increasing anthropogenic pressures. The ability of coastal lagoons, exposed to major sources of disturbance, to provide resources and suitable habitats for growth and survival of juvenile fish is especially important. We analysed three lagoons with different ecological statuses and habitat quality on the basis of their eutrophication and ecotoxicity (Trix test) levels. Fish abundances were sampled using fishing and horizontal beaming acoustic surveys with the same protocols in the same year. The relative abundance of Anguilla anguilla, Dicentrarchus labrax or the Mugilidae group was not an indicator of habitat quality, whereas Atherina boyeri and Sparus aurata appeared to be more sensitive to habitat quality. Fish abundance was higher in the two lagoons with high eutrophication and ecotoxicity levels than in the less impacted lagoon, while fish sizes were significantly higher in the two most severely impacted lagoons. This leads us to suggest low habitat quality may increase fish growth rate (by the mean of a cascading effect), but may reduce lagoon juvenile abundance by increasing larval mortality. Such a hypothesis needs to be further validated using greater investigations which take into account more influences on fish growth and recruitment in such variable environments under complex multi-stressor conditions.

Introduction

The evaluation of anthropogenic impacts on coastal lagoons requires efficient indicators of their ability to sustain ecosystem functioning and provide ecosystem services upon which human welfare depends such as protein supply. Moreover the European states have to achieve good environmental status of the EU's marine waters by 2020 and to protect the resource base upon which marine-related economic and social activities depend (EC, 2008). However, fishery–environment relationships are complex, and managing shallow water lagoon fisheries remains a difficult task (Breitburg et al., 2009; Brehmer et al., 2011a; Pérez-Ruzafa and Marcos, 2012). Fish have often been used to detect the disruption of marine and coastal ecosystems (e.g. Harmelin et al., 1995; Ramos Miranda et al., 2005; Villeger et al., 2010), and recently Rose et al. (2009) demonstrated the impact of hypoxia events on fish populations using modelling studies, while Stierhoff et al. (2006) focused on the ‘nursery quality’ of such critical habitats. More precisely, Stierhoff et al. (2009) used the RNA to DNA ratio to show that hypoxia events induce growth limitation in juvenile flatfishes, similarly as Brown-Peterson (2008) had observed with Palaemonetes pugio in an experimental study.

Since fish abundance has been recognized to be positively linked to the quality of habitats that promotes growth and survival of early life stages (Ross, 2003), the ecological status of estuaries and lagoons, offering food quantity and shelter for the larval and juvenile stages (Whitfield, 1999), is thus especially important to the sustainability of this resource. Indeed, high-quality coastal habitats, beyond their eutrophication and other physico-chemical characteristics, are also those where the abundance of juvenile fish is enhanced and fish growth optimized (Taylor et al., 2007). Using flatfish growth, size, body condition as well as population abundance, Gilliers et al. (2006) have assessed that bad habitat quality (chemical contamination) had a negative impact on biological performance.

We carried out a study in three shallow coastal lagoons in the South of France (Languedoc-Roussillon) along a gradient of habitat quality. The Languedoc-Roussillon lagoons occupy a total area of 40,000 ha, often exploited by small scale fisheries. Apart from recreational angling, the commercial fish catch amounts to ca.2000 metric tonnes per year (Guillou et al., 2002), with a total of 532 small fishing boats working in the regional lagoons. Mediterranean lagoons of this kind serve as important nursery areas for many fish including commercially important species such as Sparus aurata (Beck et al., 2001; Mercier et al., 2011). In a previous study, fishing surveys were performed to investigate fish abundances: initial findings suggested that the eutrophication level had no impact on fish diversity, even if an evidence of an impact on macrozoobenthic diversity was reported (Mouillot et al., 2005). We also assessed diagnostic estimators of the eutrophication level and ecotoxicity (Brehmer et al., 2006, 2011a) for these lagoons. The current study used an acoustic method to provide further information about pelagic fish characteristics; individual fish were detected using a fixed split-beam echo-sounder (e.g. Kubecka and Duncan, 1998a) with horizontal beaming. This paper uses data from both fishing and horizontal beaming acoustics surveys to investigate the relationship between habitat quality, defined here as the lagoon ecological status and toxicological effect of pesticides, and the abundance and size of lagoon fish.