Evaluation of impacts of bottom fishing on demersal habitats: A case study off the Pacific coast of north-eastern Japan
Introduction
With increasing attention to ecosystem-based fishery management (EBFM; Link, 2010), reducing significant adverse impacts of bottom fisheries on demersal ecosystems is a global priority, especially for protecting vulnerable marine ecosystems such as cold-water coral reefs and hydrothermal vents (Linnane et al., 2000; Eigaard et al., 2017). Many studies have investigated the effects of bottom fishing in various habitats (Thrush and Dayton, 2002), and the collapses of fish populations and demersal biota decline have been widely reported (e.g. Baum et al., 2003; Devine et al., 2006; Hiddink et al., 2017). Indeed, bottom trawl fishing, in which fishing gear frequently contacts and sweeps the seafloor, could affect a large variety of non-commercial organisms that inhabit fishing grounds as well as economically important species because trawl fishing gear is generally unselective (Davies et al., 2007). For example, the decline of biodiversity and the removal of epibenthic animals by trawl sweeping has reduced fish abundance through the degradation and loss of biogenic habitats in the waters around New Zealand and Australia (Turner et al., 1999; Smith et al., 2000). Moreover, the degradation of bottom habitats and structural changes in marine ecosystems are directly induced by the physical disturbance of gear contact (Hixon and Tissot, 2007). However, the influence of bottom fishing on demersal ecosystems may not always be negative (Hilborn, 2011). Positive effects of bottom fishing for some species have been detected in several studies; for example, light intensity trawling improved the survival of flatfishes and the survival of some invertebrate species through the removal of predators (Collie et al., 2017). In the North Sea, intensive trawl disturbance increased the feeding opportunity of plaice on polychaetes on sandy seafloors (Rijnsdorp and Vingerhoed, 2001).
The influence of bottom trawl fishing on ecosystems depends not only on fishing gear type and net-towing frequency, but also on seafloor characteristics. The vulnerability of demersal ecosystems to these physical disturbances may be specific to the type of bottom habitats characterized by seafloor depth, topography, and sediment (Thrush and Dayton, 2002; Pitcher et al., 2016). For example, the seafloor in deep-water ecosystems are expected to encounter natural disturbances less frequently than shallower ecosystems (National Marine Fisheries Service, 2005). In addition, species inhabiting stable deep-water environments generally have a longer life span and are older at maturity, which makes them slow to recover from acute reductions in abundance (Morato et al., 2006) and therefore, more vulnerable to disturbance (Engel and Kvitek, 1998).
Topographic features of the seafloor can also reflect the susceptibility of bottom habitats. For example, a seafloor with complex morphology (e.g. sloping and rough) is often utilized by many kinds of benthic species as a structural habitat and therefore, bottom topography can be used as a surrogate for the occurrence of demersal organisms (Lecours et al., 2015). In addition, convex and steep seafloors are utilized by suspension feeders such as deep sea corals, which themselves form complex structures and provide habitats to other animals (Miyamoto et al., 2017). Disturbance of seafloor structures by bottom fishing may be greater than that on a flat bottom. Thus, characterization of seafloor topography could be an essential element for assessing bottom habitat vulnerability. In addition, vulnerability also depends on sediment type because the physical damage from trawling disturbances may differ between sediment composition and is related to the recovery of species inhabiting those seafloor ecosystems (Thrush and Dayton, 2002; Hiddink et al., 2017). The existing knowledge demonstrates that the impacts of bottom fishing on demersal ecosystems depends both on gear type and fishing intensity, as well as species- and habitat-specific traits of the ecosystems, which urges us to carefully evaluate the ecosystem effects by considering multiple aspects of fisheries and ecosystems.
Within the context of EBFM, it is important to evaluate the impact of trawling on whole ecosystems including non-target species (Rice, 2014). Schemes such the ERAEF used in Australian fisheries are one example. This kind of scheme aims to provide science based information and advice on fishery management and ecosystem conservation to scientists but also to consumers and stakeholders (e.g. SH“U”N project, https://sh-u-n.fra.go.jp/shun/?lang=en; Tillin et al. (2006); Hiddink et al. (2017); Williams et al. (2011)).
These kinds of evaluation methods require substantial data with fine spatial and temporal resolutions, while in actually we often face limitations in available data, especially that which captures bottom habitat characteristics that are difficult to directly observe. The focus of the present study was to develop methods for estimating the susceptibility of seafloor habitats to bottom trawling based on bathymetry and sediment data and compare changes in species diversity among habitat types with different magnitudes of fishing impacts.
The continental shelf and slope off the Pacific coast of north-eastern Japan (Fig. 1), provides good fishing grounds for bottom trawl fisheries that target Pacific cod (Gadus macrocephalus), Japanese flying squid (Todarodes pacificus), Walleye pollock (Gadus chalcogrammus), and other demersal species. There has been an increase in catch by bull trawl and Danish seine net fisheries since the 1970s in this region (Kawauchi et al., 2018), and demersal habitats degradation made by bottom trawl fishing could cause a decline in the catch of broadbanded thornhead (Sebastolobus macrichir) by reducing shelters used to avoid predation in the northern part of the study region (Hamatsu, 2012). In this study region, scientific trawl surveys have been conducted annually to assess stock abundance of commercial species, and these combined datasets of fishery and scientific surveys enable us to assess the effect of bottom trawl fishing on demersal ecosystems. It is worth noting that the study region experienced the Great East Japan Earthquake on 11 March 2011, which interrupted fishing operation and possibly caused an abrupt shift in the demersal ecosystem structure (Narimatsu et al., 2017).
This study aimed to comprehensively quantify both the impact of bottom fishing and the response of demersal ecosystems in different habitat types to different fishing gears. We classified the seafloor into several habitat types based on bathymetry, topography, and sediment characteristics, and quantitatively estimated the susceptibility of each habitat type to physical disturbance. We then tried to evaluate the risk of bottom trawl fishing by mapping fishing frequency over the layer of habitat types. The influence of fishing impact and the evaluation method was validated with respect to the changes in species diversity collected by scientific surveys. It is also expected that the results of the present study will further provide insight for ecosystem-based fishery management.
Section snippets
Study area
The area for this study included the continental shelf and slope off the Pacific coast of north-eastern Japan (Fig. 1). The target area for the analysis was set as the area shallower than 1397 m because this depth was the maximum depth over the area where at least one trawl tow has been operated since 2006 (Fig. 2). Annual number of tows and amount of catch since 2006 for each fishing gear (e.g. otter trawl, bull trawl, Danish seine net, and small dragnet) were derived from offshore bottom
Distribution of habitat type and fishing intensity
The distribution of habitat types is shown in Fig. 4. The slope and bathymetric position index (BPI) were significantly related to PC1 and vector ruggedness measure (VRM) and hardness were related to PC2 (Table 2).
The characteristics of habitat types were also related to the abundance of demersal organisms at the survey station. For example, habitat types 2−2 where characterized by a steep slope with a complex bottom surface (Fig. 5) and higher biomass of demersal organisms especially flat
Impact of bottom fishing on demersal ecosystems
In this study, we quantified the effects of bottom trawl fishing on different seafloor habitat types and compared the response of species diversity. In previous studies, concerns of demersal ecosystems degradation caused by bottom fishing have been raised (Davies et al., 2007; Hobday et al., 2011) and thus, the influence of bottom fishing has been investigated in global marine ecosystems (McConnaughey et al., 2000; Jennings et al., 2001). The significant impacts of bottom fishing have been
CRediT authorship contribution statement
Aigo Takeshige: Conceptualization, Methodology, Software, Formal analysis, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Mai Miyamoto: Conceptualization, Methodology, Software, Validation, Formal analysis, Resources, Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Yoji Narimatsu: Validation, Investigation, Resources, Data curation, Writing - review & editing,
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This study was conducted under the SH”U“N project (https://sh-u-n.fra.go.jp/shun/?lang=en). This study was funded by the Fisheries Agency, the Ministry of Agriculture, Forestry and Fisheries of Japan and SH"U"N project. The authors greatly thank to crew of R/V Wakataka-Maru of Tohoku National Fisheries Research Institute for collecting samples and providing the datasets.
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