Precautionary management of Baltic Sea cod (Gadus morhua callaris) under different harvesting and environmental scenarios

• Main Author: Isomaa, Marleena
• Other Authors: Jörgensen, Christian, University of Helsinki, Faculty of Biological and Environmental Sciences, Department of Biosciences, Helsingin yliopisto, bio- ja ympäristötieteellinen tiedekunta, biotieteiden laitos, Helsingfors universitet, bio- och miljövetenskapliga fakulteten, biovetenskapliga institutionen, Kaitala, Veijo, Laakso, Jouni
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Helsingin yliopisto 2015
• Subjects: ekologia ja evoluutiobiologia; Gadus morhua
• Online Access: http://hdl.handle.net/10138/153132

The purpose of this thesis was to increase knowledge of heavily harvested cod stock dynamics and attain information for more suitable and sustainable management strategies. Achieving sustainable management strategies requires sufficient knowledge about the population dynamics. One tool for research this is to introduce stochastic variation models in investigated species stock dynamics. As a model species we use eastern Baltic Sea cod (Gadus morhua callarias), where the population dynamics are strongly influenced by environmental factors and fishing. We consider the stock dynamics and recovery potential under three different harvest strategies (proportional, threshold) and environmental noise scenarios. We developed age-structured discrete-time population models that include interactions with the environmental factors, e.g. salinity, fishing and natural mortality. Our results show that under current environmental conditions and threshold harvest methods cod fishing is at a sustainable level. Precautionary management ensures the best economical incomes and sustainable stocks in the long run, despite environmental fluctuations or moratoria. Results also reveal how the unpredictability of stock size, catch, and recovery increase with increasing environmental autocorrelation. Initial age structure has a strong impact on population recovery capacity in the short run, especially in over-harvested populations and under strongly fluctuating environmental conditions. Protection of younger age groups will substantially increase the recovery potential and will increase the maximum sustainable yield considerably. As a whole, precautionary harvesting strategies will increase the stock resilience against adverse environmental conditions and harvesting, guaranteeing sustainable yields in the future. Väitöskirjatyön tarkoitus oli tuottaa lisää tietoa voimakkaasti kalastetun turskan populaatiodynamiikasta sekä kannan oikeaoppisesta kalastuksen säätelystä. Kalakantojen kestävä hyödynnys edellyttää riittävää tietämystä lajin ...