KYSTFISK III. Population connectivity of cod and plaice in Danish coastal waters

Abstract

Danish fishermen have experienced a decline in coastal fishing resources and have called for an explanation of these changes. For example, cod and plaice appear to have changed their distributions with less fish present in coastal areas. The project aims at explaining this apparent strong decline in cod and plaice in coastal waters by using a range of approaches including genetic analyses of juvenile and adult individuals, statistical models of suitable habitats, numerical models linking hydrography and life history to resolve population connectivity over several years. The results produced in the project can improve our understanding of the factors driving changes in distribution and abundance of coastal fish populations and can inform assessment working groups on those species targeting the adequate management of the different stocks. Observations on hydrography and fish catches obtained from standard research cruises and monitoring stations in the Kattegat confirm the tendency of a reduction of adult cod in coastal areas in the investigated period (1995-2015). In details, while the relative abundances of the younger 1-year old cod in shallow areas (<25 m) increase, the other age groups (2 and 3 year olds) show a decline. For plaice all age groups show some increase, most prominent for the 1-year olds, which were relatively under-represented in shallow areas before 2008. Changes for plaice abundance can likely be explained by the general increase in the stocks across the entire region and adjacent areas. The detected trends in relative abundance appear to follow closely the changes in hydrography, namely an increase in temperature and a reduced salinity in the Kattegat. Hydrography variability is linked to the general circulation of the North Sea – Danish Straits – Baltic Sea system and detailed cluster analyses show the key role of the Kattegat in regulating exchanges between North Sea and Baltic. Additionally, the Skagerrak-Kattegat region has experienced significant increase in temperature (>1.5 °C since 1990) and a reduction in surface salinity, regulated by the freshening of the Baltic Sea in the same period. These results indicate substantial changes of the surface layers in the Kattegat while the deeper parts appear less affected. Since the relative changes occurring in surface and deeper layers in the Kattegat, we can infer that coastal habitats for cod and plaice are now significantly different than in the past (e.g., conditions before the 1990). We can hypothesize that changes in hydrography could elicit a behavioural response in adult fish, which could move into deeper parts and away from shallow coastal areas. The hypothesis is consistent with genetic data on juvenile cod. Indeed, the Kattegat region is composed by a mixed population of cod with North Sea origin and local Kattegat cod, while juvenile plaice appear to be predominantly locally recruited. Although the region is characterized by large year-to-year variability we detect the tendency of North Sea cod to use deeper areas than those from the Kattegat, possibly indicating different levels of adaptation and plasticity as well as differential exposure to observed environmental changes. Results from the modelling of transport of eggs and larvae of both cod and plaice are in agreement with the genetic results of local recruitment for plaice and large year-to-year fluctuations in the mixing of cod individuals. Detailed analyses of the connectivity patterns in the period 1990-2012 show no major changes in the transport of juveniles between different regions. For cod major exchanges within the Baltic sea regions support the management of this population as a single stock. On the other hand, exchanges between North Sea and Kattegat are more variable and largely correlated with inflow and outflow events in the Baltic. We conclude that changes in distributions of commercial fish stocks, such as the decrease in fish abundances especially in coastal areas observed in Danish waters, could potentially bias the estimate of stock abundance obtained through research surveys. Hence, given the complicated interactions between topographical changes, hydrography, food web and individual behaviour we recommend that better monitoring strategies for the entire stock distribution range, including coastal areas, should be developed possibly making use of modern observation technologies for autonomous habitat mapping. Overall, it is clear that the stock assessment and management challenges in such complex transition areas cannot simply be solved by adjusting boundaries for stock assessment, as movement and mixing occurs across the entire transition zone from the North Sea into the Baltic. Merging a larger area would solve the issue of movements between smaller areas, however would lose track of the dynamics and status of individual populations, which is important for understanding the stock dynamics of the species in an area, and develop appropriate management solutions. Thus, a remaining challenge is to find an optimal way to take into account the biological knowledge of stock mixing and connectivity in stock assessment and management. This challenge could be faced by supporting a holistic integrated ecosystem approach. Indeed, there is a need for integrated knowledge of connectivity across areas, focussing on stock mixing and the major drivers at fine spatial scales. A pilot study on Integrated Ecosystem Assessment (IEA) in the Skagerrak-Kattegat region could be developed through five steps: (1) Scoping management objectives and available data (2) Identify and score all drivers and pressures acting on the system (3) Define indicators for ecosystem state and track their dynamics in time and space (4) Assess ecosystem status and vulnerabilities (5) Inform fishery management and policy makers on the boundaries for the sustainable development of the area under future scenarios.