Abstract
The strength of interspecific competition and predator–prey interactionsdepends on the area of co-occurrence of the interacting species. Therefore, it isnecessary to quantify the changes in the spatial overlap of trophically connectedspecies to understand the outcomes of species interactions. In the Baltic Sea, theinterplay between cod and flounder has previously been neglected. In this study, we usefour decades of data on cod and flounder distributions covering the southern and centralBaltic Sea to: (1) model and map the changes in the distributions of the two speciesusing generalized additive models; (2) quantify the temporal changes in the potentialcompetitive and predator–prey interactions between them using spatial overlap indices;(3) relate these changes in overlap to the known dynamics of the different cod andflounder populations in the Baltic Sea. Competition overlap has continuously increasedfor cod, from the beginning of the time-series. This is a possible cause of the observeddecline in feeding levels and body condition of small and intermediate sized cod.Flounder overlap with large cod instead has decreased substantially, suggesting apredation release of flounder, potentially triggering its increase in abundance anddistribution range observed in the last decades.
Highlights
The strength of interspecific competition and predator–prey interactions depends on the area of co-occurrence of the interacting species
Quantifying the spatial dynamics of species distributions has proven to be a central aspect for understanding the outcomes of species interactions, including how spatial heterogeneity affects functional responses, or how the results of small-scale experimental studies on trophic interactions can be extrapolated to heterogeneous natural systems (Chesson, 1998; Englund & Leonardsson, 2008)
Standardized catch per unit of effort (CPUE) in weight of cod and flounder were obtained from a database containing data collected during the Baltic International Trawl Survey (BITS; ICES 2014) and historical Swedish and Latvian trawl surveys carried out between 1979 and 2016 in ICES Subdivisions (SDs) 22–29 (Fig. 1, Online Resource Appendix 1, Fig. A1, Table A1)
Summary
The strength of interspecific competition and predator–prey interactions depends on the area of co-occurrence of the interacting species. Changes in interspecific interactions through predation and resource competition can affect for example abundances, spatial distributions, mortality rates, growth, fecundity, and age/size at first maturity (Sparholt, 1994; McPeek & Peckarsky, 1998; Klanderud, 2005; Tylianakis et al, 2008; Hillyer & Silman, 2010) In aquatic systems, these effects may in turn produce massive reorganizations of the food webs by means of trophic cascades resulting in regime shifts (Gardmark et al, 2015; Rudman et al, 2016; Walsh et al, 2016), with serious implications for species conservation and the management of exploited populations. For example, the distribution maps obtained from these models have been used to estimate overlap between species, populations and life stages, which can in turn be used to infer interaction strength (Hunsicker et al, 2013)
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