Trophic overlap between expanding and contracting fish predators in a range margin undergoing change

Mats Westerbom,Olli Mustonen,Antti Lappalainen,Alf Norkko

doi:10.1038/s41598-018-25745-6

Mats Westerbom, Olli Mustonen + Show 2 more

Open Access

https://doi.org/10.1038/s41598-018-25745-6

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Abstract

Climate change is predicted to cause a freshening of the Baltic Sea, facilitating range expansions of freshwater species and contractions of marine. Resident marine flounders (Platichthys flesus) and expansive freshwater roach (Rutilus rutilus) are dominant consumers in the Baltic Sea sublittoral where they occur in partial sympatry. By comparing patterns of resource use by flounders and roach along a declining resource gradient of blue mussels (Mytilus trossulus) our aim was to explore predator functional responses and the degree of trophic overlap. Understanding the nature of density-dependent prey acquisition has important implications for predicting population dynamics of both predators and their shared prey. Results showed a highly specialized diet for both species, high reliance on blue mussels throughout the range, similar prey size preference and high trophic overlap. Highest overlap occurred where blue mussels were abundant but overlap was also high where they were scarce. Our results highlight the importance of a single food item - the blue mussel - for both species, likely promoting high population size and range expansion of roach. Findings also suggest that range expansion of roach may have a top-down structuring force on mussels that differ in severity and location from that originating from resident flounders.

Highlights

Investigating the causes and consequences of shifts in species range boundaries is a key ecological undertaking in a world facing increasingly large-scale ecosystem change
We examine the diet of flounder (Platichthys flesus) and roach (Rutilus rutilus), two partially co-occurring fish species in the shallow Baltic Sea sublittoral
Predation on blue mussels has been considered unimportant in a Baltic Sea context

Summary

Introduction

Investigating the causes and consequences of shifts in species range boundaries is a key ecological undertaking in a world facing increasingly large-scale ecosystem change. Monitoring data since the 1990s shows that the abundance of roach in outer coastal waters has increased by more than an order of magnitude whereas numbers of flounders have concomitantly declined dramatically[17,19] (see Supplementary Fig. S1, unpublished national monitoring data by the Natural Resources Institute) This change has taken place with a parallel decline in sea surface salinity. Alongside with a geographical shift, there has been a regional range expansion of roach and other cyprinid species towards more open outer archipelago areas that previously were dominated by marine fish –such as flounders[20] The reason for this shift in dominance is believed to be eutrophication, and ongoing changes in climate favour roach and other cyprinids that benefit from higher water temperatures and declining salinity caused by a change in climate[18,21]. A high similarity between coexisting species may lead to competition and resource partitioning especially when commonly used resources become sparse[24]

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