Sea -based aquaculture sites attract a wide range of wild marine species (Barrett et al. 2019). These species can either be attracted to the shelter provided by the marine infrastructure, or to wastes discharging from the fish farm, such as feed or excrements. Artificial habitats in natural environments can disrupt the natural behavior of the attracted species . For instance, the organisms may prefer the artificial habitat over the natural one, even if this means that it leads to impaired reproduction, altered body conditions and reduced survival (Dempster et al. 2011, Tanner & Williams 2015). The existence of such ecological traps has been well documented in terrestrial systems, however, the studies on ecological traps in the marine environment is limited.
Previous studies on the interaction between intensive aquaculture and wild fish have been very much focused on pelagic species occurring in immediate proximity to the net pens (Dempster et al. 2009). However, benthic impact assessment has suffered from the lack of a holistic approach, with very few studies focusing on the effects of aquaculture on larger (epibenthic) biota in greater depths (below 50 m).
The aim of this ongoing study i s to examine the effect of aquaculture on demersal fish living in temperate deeper waters. Specifically, the goal is to provide a systematic overview of the fish species visiting the seafloor beneath fish farms and explore differences between aquaculture locations , seasons and production status along Norwegian fjords.
Material and methods
Stereo baited remote underwater videos systems (BRUVs) consisting of a metal frame with two cameras, lights and a bait container (Fig. 1) were deployed from November 2021 to March 2023 at 10 aquaculture locations in Storfjorden and Romsdalsfjorden in Møre and Romsdal county (Norway) . Of 51 BRUV deployments 19 were taken at farms with ongoing production , 9 were taken at farms during a fallow period of minimum 2 months and 23 were taken at c ontrol areas (ca. 1 km away from farming site), uninfluenced by the fish farm activities and discharge. BRUVs were lowered onto the seabed (75 – 200 m ) and stereo videos were recorded for 60 minutes.
The video recordings were systematically analyzed to extract species identification and counts (maximum number of finfish present in the field of view of the cameras , MaxN ) (Watson et. al 2010). As a measure of fish density in the vicinity, the parameter “time to the first appearance of finfish” was used. Measurements were conducted using the EventMeasure software by SeaGIS (20 21). Th e effect of fish farms and certain environmental factors on the demersal fish community will be tested with hierarchical multivariate statistics. Specifically, it will be tested if fish farms itself (farm vs. control) or their production status (production vs fallow period) have an effect on finfish abundance, presence and community composition across seasons.
Data of the experiment is still being analyzed. Preliminary results indicate that the seafloor in around 100m under aquaculture sites is visited by a rather high diversity of finfish species, albeit infrequently. Across all different farming sites, a total of 15 different finfish species were observed, of which respectively 10 species were observed at farms with ongoing production, 10 at control areas and 14 at fallow farms. An average of 2.7 (SD 1.8) finfish were observed within a 60min deployment.
As a first statistical trend, no significant influence of fish farms on the abundance or presence of demersal finfish close to the bottom under deep fjord aquaculture sites. Both the site (fish farm vs. control) and status (production vs. no production) do not seem to have any significant impact on species composition, presence , and abundance.
Although aquaculture sites presumably have a huge environmental effect on their surroundings, t he study indicates that occurrence and abundance of benthic demersal fish at a depth of 75 -200m is not affected by aquaculture farming activities. Furthermore, it showcases that BRUVs offer a low-cost alternative to diver operated observations and have great potential not only in shallow, tropical but also in deeper temperate waters . However, the influence of light and associated bait on attracting finfish and leading to biased estimates needs to be further investigated.
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