What is the significance of cormorant predation on perch in coastal areas? Often basic data such as fish population size and structure are lacking or are incomplete making the effect of cormorant predation on natural fish populations difficult to estimate. The latest Editor's Choice paper from Veneranta, Heikinheimo, and Marjomäki set out to evaluate this in the area of the Quark in the northern Baltic Sea. The study area was seen as an optimal location for predation estimation due to the rapidly increasing large population of breeding cormorants, a viable commercial and recreational fishery, and an abundant perch population.
The rapid increase in the cormorant population has led to a debate, both in public discussion and in scientific literature, about the role of cormorants in the coastal ecosystem and especially their potential impact on fish stocks and catches. One of the issues relating to cormorant protection and fisheries has been the measurement and evaluation of cormorant predation effect.
To estimate the cormorant predation mortality on a fish population in small scale, the authors tagged perch (Perca fluviatilis) near spawning time with small PIT (Passive Integrated Transponder) tags under dorsal skin. Cormorants regurgitate the hard parts of fish in their breeding colony or other locations like roosting sites. After breeding season, the tags were scanned from colonies. The proportion of tags found is the minimum estimate of cormorant predation mortality on perch.
The tag recapture results were further used to estimate the cormorant-induced mortalities that can be used, for example, in wider scale in fisheries yield models. On the basis of the mortality estimates, a yield-per-recruit model was constructed to estimate the potential maximum cormorant predation effect on perch stocks in an assumed equilibrium state.
At a local scale, within the area of most intensive predation by cormorants, the long-term maximum loss of perch yield could be remarkable - but it dilutes when results are extended to larger areas. Dependence of predation rate on the prey density as well as density dependence in perch growth and mortality partly compensates the cormorant impact on perch stocks. The results emphasize the consideration of spatial scale in management of cormorants and fisheries and are a first attempt to use a tagging experiment to estimate the potential effects on perch yields and stocks.
"A key strategic approach throughout the cormorant effect assessment was to first estimate the uncertainty about the real value of every input-variable and then combine these uncertainties to understand the uncertainty in output-variable estimates", states Veneranta, "The results are thus presented as probability distributions to communicate the reliability of our current best knowledge to the stakeholders in in policy formulation and decision making".
Read the full paper in ICES Journal of Marine Science.
The PIT tags from cormorant-consumed perch were scanned from cormorant colonies after breeding season. In total, 10% of tags were found close to nests.