Can Cormorants Be Used As Indicators of Local Fish Abundances?
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Can cormorants be used as indicators of local fish abundances? A diet study of cormorants on Gotland Oliver Hansen Degree project in biology, Master of science (2 years), 2021 Examensarbete i biologi 45 hp till masterexamen, 2021 Biology Education Centre and Institutionen för biologisk grundutbildning vid Uppsala universit, Uppsala University Supervisors: Richard Svanbäck and Karl Lundström External opponent: Alessandro Culicchi Abstract Human wildlife conflicts can represent missed opportunities for ecological monitoring, including tracking invasive species. The great cormorant Phalacrocorax carbo sinensis is the centre such a conflict, where the lack of concrete scientific evidence is often replaced by anecdotal evidence, leading to the vilification of these birds. The primary aim of this study was to assess the extent of the overlap between cormorant diet and the fish the fishermen are allowed to catch on the North West coast of Gotland, the Baltic seas´ biggest island. To assess cormorant diet, the otoliths in the cormorant pellets were analysed. Secondary aims included assessing the potential to use cormorant diet as a proxy for local fish abundances by comparing it to monitoring fisheries in the same area. Highly contentious species only included cod, herring and flounder, none of which were commonly consumed by cormorants. Cormorants and the monitoring fisheries found comparable proportions of all species except for flatfish herring sprat, sculpin. We conclude that the cormorant poses a relatively low risk to the fishing industry on the North Western coast of Gotland and that they could potentially be used as a sentinel for local fish abundances, including tracking invasive species such as the round goby. Introduction Conflicts between living organisms occur when there is a finite pool of shared vital resources. Through innovation and exploitation efficiency, humans have now become a major ecological force (Waters et al. 2016). As a dominant force, we often see wildlife as stealing resources that should ‘rightfully’ belong to us. Such conflicts between wildlife and us are called human- wildlife conflicts. These conflicts tend to be very complex, both on the ‘human’ and ‘wildlife’ side of the equation. On the wildlife side, the severity of the conflict will depend on the size of the animal, its behaviour, the population size and the shared resource in question. The relationships between these factors and the severity of the conflict are not linear, but rather dynamic. For example, sensitizing macaques through hazing modified their behaviour and reduces conflict, without impacting population size (Honda et al. 2019). Thus, we cannot make blanket statements such as ‘large animals with large population sizes will be the cause of the most severe human-wildlife conflict’. On the human side, we have a similar situation. The conflict between human actors can be as severe and challenging to solve as the conflict between humans and wildlife itself (Rauschmayer & Behrens, 2008), because often scientific judgment is replaced by emotional responses, especially if scientific evidence is lacking and livelihoods or hobbies/passions are at stake (Carss D et al. 2009). In addition, social factors such as religion, ethnicity and affiliation further complicate human wildlife conflicts, as these play a role in determining the outcome of these conflicts (Dickman 2010). One of the most polarizing birds in Europe is the great cormorant (Phalacrocorax carbo sinensis), henceforth cormorant. This generalist piscivorous predator employs both social and solitary fishing strategies, both on pelagic and benthic prey and in fresh and salt water (Grémillet et al. 1998, Grémillet et al. 2004). Their vision is surprisingly bad (White et al. 2007) for an animal who achieved the highest known foraging yield for a marine predator (Grémillet et al. 2004). Uncommonly for pelecaniform birds, cormorants can nest almost anywhere, only requiring protection from ground predators and a structure large enough to support the nest and its inhabitants (Causey & Padula 2019). Cormorants have a day roost for resting and digesting and night roost for sleeping. They are both are in close proximity to their breeding and feeding grounds (Causey & Padula 2019). A stark population increase in Europe has raised fears of negative impacts on aquatic as well as terrestrial ecosystems (Klimaszyk & Rzymski 2016). Like in many parts of Europe, the Swedish population of cormorants has increased from 100- 150 pairs in 1965 (Havs och Vatten myndigheten 2015a) to 40 000 in 2012 (Naturvårdsverket 2013). The exact reasons for this expansion are unclear. It seems that the cormorant population is benefiting from a number of factors. They are protected by law, no longer limited by food supply (Carss DN & Marzano 2005), favoured by environmental change (Marzano et al., 2013) including benefiting from reduced environmental toxins such as DDT and PCB and have gained a novel food source in invasive species such as the round goby (Neogobius melanstomus) (Bzoma & Meissner 2005). Although evidence suggests cormorants inhabited Sweden 9 000 years ago, they probably did not breed in Sweden, or at least were not very common, during the 17th and 18th century (Engström 2001a). Cormorants were present in Sweden during the 19th century (Engström 2001a) and human persecution in the form of nest destruction and hunting has led to their local extinction within Sweden in the beginning of the 20th century (Ericson & Carrasquilla 1997). It is also though that pollutants such as DDT and PCB contributed to their decline (Hermann et al. 2018). Once protective measures were in place, including the ban of said pollutants, the number of breeding pairs started to increase in the latter half of the 1970´s and cormorants subsequently recolonized the Baltic in the 1980´s (Hermann et al. 2018). On Gotland, evidence for a subspecies of the Great cormorant breeding is 2 500-5 000 years old (Ericson & Carrasquilla, 1997). The first breeding pairs since local extinction were found in 1992 and the population grew to 10 500 breeding pairs in 2008, declined to around 8 000 pairs from 2010- 2013, increased to around 10 000 pairs in 2014 and has been declining slowly since then to around 7 500 pairs in 2018 (Hermann et al. 2018). Cormorants have caused elevated nitrogen, ammonium and phosphorous levels in at least one historically nutrient poor lake on Gotland, Ajkesträsk, at which hunting has now been allowed, as the protection of the lake outweighs the protection of the cormorant according to Länsstyrelsen Gotlands Län (2020). Diets of generalist and opportunist carnivores have been said to be a good indicator of temporal as well as spatial variation in prey availability (see Moorhouse-Gann et al. 2020), therefore, predator diet could possibly be used as proxy for prey abundances. Combining seabird diet and fish monitoring data is likely to reflect a more accurate state of nearshore fish recruitment (Robinette et al. 2018) and changes in cormorant diet has been shown to coincide with similar changes in fish community (Boström et al 2012). Mecenero et al. (2007) suggested that seal diet could be a proxy for future fish catches. One way to assess the diet of cormorants is based on the pellets that they regurgitate. The pellets are the undigested remains of fish (mostly bones and corneas) covered in a membrane. Pellets are a suitable method of assessing diet because most hard parts such as spinal bones but also ear bones, also known as otoliths, remain identifiable. Pellets easy to find and non-invasive to collect (Duffy and Laurenson 1983). Although cormorants are a highly mobile species being able to migrate from the Baltic Sea to the Mediterranean Sea (Ericson & Carrasquilla 1997), this should not impact our ability to predict local fish abundances based on their pellets. During the breeding season, cormorants seem to stay within 25km of their nesting site (Paillisson et al. 2004), although foraging at a distance of 35km from the nest has also been recorded (Gremillet 1997). The feeding range of cormorants seems to also stay fairly limited to the nest site when fish abundances are declining, and respond with increased number of foraging trips as opposed to fewer, but further trips (Gremillet 1997). However, Gremillet (1997) also postulates that cormorants will commit to further feeding trips if a more energetic resource is available there. In addition, cormorants are constrained by not having waterproof feathers and thus need to stop and dry after being in water (Srinivasan et al. 2014). Because of this drying, the distance they can travel between meals is reduced. Therefore, a pellet represents the diet within the feeding range of the cormorant. Cormorants have the ability impact local fish communities thorough both top down (predation) and bottom up (eutrophication) (Piotr et al. 2015, Gagnon et al. 2015). Thus a growing population of cormorants has caused a polarizing debate between those who deem this a conservation success and those who see cormorants a nuisance to ‘their’ fishing industry (Žydelis & Kontautas 2008, Bregnballe et al. 2015). This debate is known as the ‘cormorant problem’ (Marzano et al. 2013b). Cormorants have a bad reputation as a bird that consumes vast amounts of fish and this can lead to a perceived conflict, when in reality the conflict is likely to be less severe than public perception would suggest (Žydelis & Kontautas 2008). Other factors contributing to the bad reputation include damaging trees (Petersson 2020), causing unpleasant odour, eutrophication, and being seen as invasive species. This however does not mean that they have no effect, but their impact is exaggerated, with speculation substituting evidence as the basis of arguments against cormorants. The problem is that we more often than not do not know the impact cormorants have on the local environment. Cormorants have been known to catch between 3-13% of total valued commercial and recreational catches and between 10% and 44% of fish biomass available in certain areas (Östman et al.