Effects of a large northern European no‐take zone on flatfish populations^a

Abstract

In March 2006, a 360 km² no-take zone (NTZ) was established north of Gotland in the central Baltic Sea, with the purpose to scientifically evaluate the effects of a fishing ban on flatfish populations. A monitoring programme was set up to study the populations in the NTZ and in a reference area east of Gotland where the fishing pressure was high. The programme included fishing with multimesh survey nets, modelling of potential larval export and estimation of fish consumption by large marine predators. Overall, the results showed a clear positive effect of the NTZ on turbot Scophthalmus maximus, with higher densities in the closed area compared with the fished area and also higher densities after closure compared with before. The NTZ also had older individuals and a more even sex ratio. This, in combination with a high potential for larval export from the NTZ to Gotland, shows that the marine reserve may be important for maintaining a viable S. maximus stock at Gotland. Also, for flounder Platichthys flesus, the densities were higher in the NTZ compared to the reference area and there was a net larval export to the fished area. For both species, density-dependent growth was evident, with a lower length at age in the closed area. Potential predation by grey seal Halichoerus grypus and great cormorant Phalacrocorax carbo sinesis on flatfishes, that could hamper the evaluation of the marine reserve, was also addressed. Taken together, the results show that there are clear benefits of the fishing ban for both flatfish species within the NTZ, while the net effects on fisheries are difficult to quantify.

Inferring spatial structure from population genetics and spatial synchrony in demography of Baltic Sea fishes: implications for management

“…Coastal flounder was the only coastal species with no apparent genetic structure, likely a result from drift and mixture of larvae from spawning grounds to nursery and feeding grounds in coastal areas (Florin et al . ).…”

“…The flatfishes, plaice and turbot, likely have high gene flow due to high mobility of larvae (Florin et al . ; Moksnes et al . ).…”

“…For these two species, genetic drift seems to override gene flow among discrete areas where spawning occurs, suggesting that specific management units can be identified. Coastal flounder was the only coastal species with no apparent genetic structure, likely a result from drift and mixture of larvae from spawning grounds to nursery and feeding grounds in coastal areas (Florin et al 2013).…”

“…The other offshore species (herring, plaice, turbot and three-spined stickleback) had low F ST and weak IBD, that is based on neutral genetic markers they were classified as having a single population in the Baltic Sea. The flatfishes, plaice and turbot, likely have high gene flow due to high mobility of larvae (Florin et al 2013;Moksnes et al 2014). Reiss et al (2009) also found (in a review over whole NE Atlantic) little genetic differentiation in most flatfish species, despite evident homing behaviour.…”

Fecundity regulation by atresia in turbot Scophthalmus maximus in the Baltic Sea

“…According to observations in the present study, large differences in realized fecundity between areas may occur, and potentially also between years. Hence, in addition to varying abiotic conditions affecting offspring survival prior to settlement (Nissling et al, 2006) and food availability affecting growth and potentially survival after settlement in nursery areas (Martinsson, 2011), variability in egg production according to fish condition may contribute to the high recruitment variability of S. maximus in the Baltic Sea (Molander, 1954;Florin et al, 2013). A topic for future studies is to evaluate the interannual variability of the realized fecundity in different areas, and to reveal whether differences are driven mainly by discrepancies in the potential fecundity, i.e.…”