Abstract
Environmental management of coastal aquaculture is focused on acute impacts of organic and nitrogenous wastes close to farms. However, the energy-rich trophic subsidy that aquaculture provides may create cascades with influences over broader spatial scales. In a fjord region with intensive fish farming, we tested whether an ecosystem engineer, the white urchin Gracilechinus acutus, was more abundant at aquaculture sites than control sites. Further, we tested whether diets influenced by aquaculture waste altered reproductive outputs compared with natural diets. Urchins formed barrens at aquaculture sites where they were 10 times more abundant (38 urchins m-2) than at control sites (4 urchins m-2). Urchins were on average 15 mm larger at control sites. In the laboratory, urchins fed aquafeed diets had 3 times larger gonad indices than urchins fed a natural diet. However, their reproduction was compromised. Eggs from females fed an aquafeed diet had 13% lower fertilisation success and 30% lower larval survival rates at 10 d compared with females fed a natural diet. A reproductive output model showed that enhanced numbers of 10 d old larvae produced by the dense aquaculture-associated aggregations of G. acutus will supersede any detrimental effects on reproduction, with larval outputs from aquaculture sites being on average 5 times greater than control sites. The results show that aquaculture waste can act as a trophic subsidy in fjord ecosystems, stimulating aggregations of urchins and promoting the formation of urchin barrens. Where finfish aquaculture is concentrated, combined effects on the wider environment may produce ecosystem-level consequences.
Highlights
Trophic subsidies occur via the flow of energy between ecosystems, as both matter and organisms (Larsen et al 2016)
We investigated the effect of aquaculture on population densities of wild G. acutus and tested the physiological and reproductive consequences of consuming an aquaculture-derived trophic subsidy
In a region of western Norway with intensive Atlantic salmon farming (Masfjorden; Fig. S1 in the Supplement at www.int-res.com/articles/suppl/q010 p279_supp.pdf), we tested whether abundance and size of the white urchin Gracilechinus acutus differed between aquaculture and control sites
Summary
Trophic subsidies occur via the flow of energy between ecosystems, as both matter and organisms (Larsen et al 2016). While trophic subsidies occur naturally, increasingly, anthropogenic subsidies are driving changes to ecosystems and food webs by altering the distribution, abundance, growth and reproduction of consumers in recipient environments (Marczak et al 2007, Oro et al 2013). Less well understood are the impacts of aquaculture subsidies on a broader scale, where waste may be delivered in quantities more readily assimilated by the wider ecosystem, with potential consequences for the marine food web (Bannister et al 2016, Broch et al 2017). Wild marine fauna, including fish (FernandezJover et al 2011b) and mobile invertebrates (Olsen et al 2012, White et al 2017) consume aquaculture waste. Potential repercussions to the fitness of individuals and dynamics of populations that receive aquaculture-derived trophic subsidies remain unexplored, as are the mechanisms through which subsidies could cascade through ecosystems on a broader scale
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