Written by Zuzanna Dusza
Fisheries are critical to supplying a large proportion of the world’s animal protein, with demand for fish products increasing annually. As such, well maintained, sustainable and efficient fisheries and aquaculture systems will be crucial in order to continue meeting this demand in the future. However, it is clear that sustainable management strategies are currently not in place across most of the industry. This is obvious when you look at bycatch, the unintentional catch of non-target species, which has been revealed to constitute 40.4% of worldwide marine catches.
Due to the negative link between bycatch and marine ecosystems, including population declines and disruption of food web interactions, several attempts have been made to reduce the problem. One of the most effective ways to do this is to decipher target and bycatch species’ sensory systems to create mitigation devices to maximise catch without compromising vulnerable species. There exist many fishery and aquaculture-based examples which highlight substantial knowledge gaps and insufficient research, but also the huge potential for integrating sensory ecology into fishery practices. In order to ensure efficiency and sustainability are both achieved, this area of research has to be a priority.
In recent efforts to tackle the issue of bycatch in fisheries, several strategies aiming to integrate animals’ sensory systems into deterrent gear have been explored. Unlike us humans with our five basic senses, marine creatures take sensory systems to another level with other forms including electroreception in sharks and rays and magnetoreception in some fish and whales. Whereas we are very visual creatures relying heavily on sight, for most marine creatures the dominant sense is usually something else like sound, smell or other tactile senses.
To target the problem in the gill net industry, one strategy proposed in a 2015 paper consists of visual stimulus panels to deter all potential bycatch taxa. These panels exhibit high internal contrast and consist of a grated and chequerboard pattern model (see image above). The idea behind these is to ingrate them into the gill net at 2 metre intervals and for the panels to act as a repellent to non-target species such as sea turtles, cetaceans and seabirds. Sounds great right? As much as this is a relatively new technique that requires more testing, a pinch of scepticism should also be applied when considering how universal these panels would actually be. While taxa such as seabirds are known to be visually guided, this approach could prove to be too simplistic when considering pinnipeds (seals) and cetaceans (whales and dolphins) which tend to rely on senses other than vision, especially in turbid conditions. This may well mean these panels are too simplistic, yet this requires further research in the field to determine for sure.
Another area which has been explored, but which still calls for much further research is that of fish aggregating devices (FADs). These are manmade objects (often buoys and floats) which are used to attract fish species such as tuna and marlin and are widely used to maximise catches in the purse seine industry. A 2004 paper highlightsed FADs as an important area to explore in terms of fish sensory systems, but also points to the varying degree in which the different systems have been researched. Narrowing down which senses fish use to queue in to FADs would allow these devices to be tailored specifically to the target species and help avoid catching the non-target species who are attracted by the abundance of fish.
Aquaculture, the artificial rearing of marine organisms, is the fastest growing sector of food production in the world. However, in order to achieve its potential, it needs to be managed sustainably and efficiently. Infection of Atlantic farmed salmon by parasitic sea lice is an industrial scale problem affecting both farmed and wild stocks and has a great economic impact. So far solutions such as infection pressure monitoring, delousing species and co-culture with cleaner wrasse have been proposed, but these solutions are unsustainable and inefficient. For example, cleaner wrasse are unsustainably harvested from wild ecosystems and are ineffective in cold water.
To replace these management strategies, a report in 2005 suggested applying the knowledge of louse sensory systems with focus on the host-associated sensory stimuli that may be used by parasites to recognise a host. This would be done in hope of disrupting detection of host location and subsequently settlement by directing the parasite’s free-living stages into swimming away from the salmon cages and migratory routes of wild salmon. Multimodal sensory integration is said to play a role in louse host detection with various cues such as decreased light intensity due to shadows cast by swimming fish, as well as olfactory cues acting as signals. Understanding these mechanisms could allow for the modification of farmed salmon environments and decrease mortality rates.
When considering aquaculture, it is also critical to mention the issue of animal-human conﬂict as a consequence of competition for ﬁsh between marine predators and ﬁshermen. This problem is well exempliﬁed in Scotland, home to the largest and continually growing aquaculture of Atlantic salmon, globally. Here, as stated in a 2016 study, grey and common seals exist in conﬂict with ﬁshery farms, speciﬁcally those of Atlantic salmon and sea trout. As of 2010, with the introduction of a seal licensing system, seals are now permitted to be shot and killed “to protect the health and welfare of farmed ﬁsh”. This introduction of legal shooting practices has consequently raised concerns over seal population welfare and alternative protection measures. Hence, among other solutions, increased use of non-lethal acoustic deterrents, or pingers, has been proposed.
So far, the use of pingers to deter seals has been reported to have a mixed eﬀect, with some ﬁsh farmers reporting seals becoming habituated to pingers after a few years of use, and others even suggesting pingers to present as an attraction to certain individuals. These responses highlight the problematic knowledge gaps existing around speciﬁc animals’ sensory systems and ﬁlling those gaps through more experiments could help ﬁne-tune deterrent gear to function as eﬀectively as possible. As it stands, despite a 52% increase in seal-scarer use from 1985 to 2001, as reported in 2004 survey, only 23% of 92 questioned ﬁshery managers considered pingers eﬀective in Scotland. Perhaps a device more directed at pinnipeds’ predominant tactile sense mediated by their vibrissae should be considered for future research.
Looking at all the various studies working to apply sensory ecology to the fishery and aquaculture industries, it becomes apparent just how problematic the endeavour is. Knowledge gaps and insufficient evidence make it very difficult to come to a universal conclusion and to design gear which reduces all types of bycatch while simultaneously maximising target species catch. However it is also clear that applied knowledge of sensory ecology could help with the design of more efficient aquaculture systems, which would increase animal welfare standards and reduce costly losses.
Unfortunately, current management lacks the use of sensory ecology to inform policies and I strongly stand by the opinion that guesswork measures will no longer suffice. Major efforts to make practices more sustainable and a paradigm shift in the global fishery and aquaculture industries is required. Realistically, in order for this to occur, costs of redirecting monetary resources into research and gear modifications need to be offset by benefits and uncompromised catch. As discussed, this poses problems on many fronts, however, I believe that if we are to continue exploiting the ocean’s resources at current and projected rates, this must be done in a sustainable manner whereby the gear we deploy is selective, bycatch and discards virtually non-existent and the welfare of aquaculture reared fish high.
Zuzanna Dusza is a marine biology student at the University of Exeter. She is passionate about the natural environment, especially the marine habitat and all organisms that inhabit it. She’s planning to pursue a career in science communication and writes articles for a Cornwall-based environmental magazine “Bloom in Doom”. For media enquiries you can contact her via her LinkedIn profile here.
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