Epsilon Archive for Student Projects

Abstract

Marine heatwaves (MHWs) are increasing globally in both frequency and intensity. Despite the potential effects on ecological and economic values, little is known about MHW's effect on fish body growth. This is especially the case for shallow coastal areas, including those in the Baltic Sea. Body growth in fish is known to be affected by gradually increasing temperature and consistently higher temperatures, with small and young individuals often growing faster but reaching a smaller asymptotic size in warmer waters. In this study, I investigated if MHWs affects (i) fish body growth, and (ii) if such effects vary with size and age, and (iii) if size at catch was also affected similarly. This was done using two measurements of MHWs: years with one or more MHW occurring and the yearly total cumulative intensity of MHWs. Using these two measurements I also investigated if MHWs had increased or decreased since the year 2000 in my coastal study sites. I used monitoring data, which included temperature and catch data of European Perch (Perca fluviatilis) from sites along the shallow waters of the Swedish east coast. My results indicated that MHWs occurred in all sites. However, I found no increase or decrease in any of my MHW measurements since the 2000s. With some differences depending on the specific measure of MHWs used, MHWs had an overall positive effect on body growth that varied with age and size. Similarly, MHWs had a positive effect on size at catch. Interestingly, during years with one or more MHW the positive effect on growth decreased with age and size. This is in line with how many fish, including perch, respond to gradually warming temperatures. Whether the effects of MHWs will continue to be positive for perch body growth in the future remains unknown as the frequency and intensity of MHWs are expected to increase. The findings in this study highlight the presence of MHWs in shallow coastal waters and its potential to body growth of Baltic Sea fish.