The flounder in the Baltic was important species from both the ecological and economic aspects over entire XX century. Historically, flounder catches reached maximum in 1930' (about 60 thousand tons) in very short time and rapidly collapsed at the beginning of 1940s. After that period, catches fluctuated, but stayed at relatively low level in comparison to 1930. There is an ecological puzzle - what happened to flounder stock that catches increased to so high level and collapsed so rapidly next ? Was that an environmental factor or a human impact ? To answer that question, the first step is to reconstruct Baltic flounder stock biomass.
Flounder is assessed by the Baltic Fisheries Assessment Working Group, but it belongs to category "data-limited stocks" and so far there is no accepted quantitative assessments of flounder. There are 4 flounder stocks in the Baltic Sea; one with demersal-spawning flounder and three with pelagic-spawning. These two groups differ in their spawning habitat and eggs characteristic. Each of them needs different environmental conditions for spawning. Demersal eggs require first of all sufficient oxygen concentration, so the spawning area for those fish will exclude places where there is hypoxia condition at the bottom, while flounder with pelagic eggs needs appropriate salinity level to obtain the neutral egg buoyancy. There are also population differences within the pelagic-spawning flounder, which has led to the definition of three assessment units.
The aim of this project is to perform the reconstruction of the biomass dynamics of Baltic Sea flounder in the light of environmental conditions. To achieve this, the historical catch data and environmental conditions characteristics will be compiled and analysed. Long term datasets are considered as very useful to increase our knowledge on stock and ecosystem dynamics. They usually contain different combinations of natural conditions and human pressures, which give a lot of information to help predict the biomass and recruitment fluctuations.
In this project we will explore historical catch data back to 1906 (Hammer at Feistel et al.,2008) and model stock dynamic taking into account all available historical information (e.g. flounder reproductive volume – Ustups et al., 2013).
The main product of this project will be biomass reconstruction of flounder across the 100 years period, including the discussion of factors that could influence its dynamics.
Feistel, Rainer, Günther Nausch, and Norbert Wasmund. "State and evolution of the Baltic Sea, 1952–2005." A detailed (2008).
Ustups, Didzis, et al. "The influence of environmental conditions on early life stages of flounder (Platichthys flesus) in the central Baltic Sea." Journal of Sea Research 75 (2013): 77-84.
Gustafsson, Bo G., et al. "Reconstructing the development of Baltic Sea eutrophication 1850–2006." Ambio 41.6 (2012): 534-548.