Assessment of the effects of oil exposure on the population dynamics and abundances of Atlantic cod and haddock using state-space models

The objective of this project is to assess the potential ecological impacts of oil pollution on two economically important fish stocks in the Norwegian and Barents Seas, the Northeast Arctic (NEA) stocks of Atlantic cod (Gadus morhua) and haddock (Melanogrammus aeglefinus), through simulating potential effects of early life-stage mortality (eggs/larvae) on stock dynamics and abundances.

Background: The Norwegian-Barents Sea is an area with regular oil traffic and of increasing interest for oil production, including areas close to important spawning grounds of the two stocks off the Lofoten and Vesterålen islands, where oil exposure could negatively impact the early life-stages of these fish.

Approach: We will develop a Bayesian state-space models for the population dynamics of NEA cod and haddock using catch statistics and scientific survey data for all life-stages, from eggs to spawners. This approach allows us to incorporate prior knowledge on model parameters (Bayesian framework), and to account for inaccurate sampling (observation error) and environmental stochasticity (process noise).

Expected results: The models will be used to simulate the effects of different levels of early life-stage mortality on the dynamics and abundances of the two stocks under various assumptions about climate conditions and fishing pressure.

Relevance: The expected results are important for successful management of these stocks, particularly in the Lofoten-Vesterålen area where oil production and increased oil traffic are projected close to major spawning grounds of NEA cod and haddock.

Lay summary

The project was completed 13. November 2014. 

One of the main concerns regarding the extraction of petroleum resources in marine habitats is the potential effect of accidental oil spills on important ecosystem services such as fisheries. For instance, petroleum extraction around the Lofoten and Vesterålen islands in Norway is highly debated, because economicallyimportant fish species such Atlantic cod reproduce in this area. We investigated how long-lived marine fish species such as cod might respond to severe mortality events caused by acute oil exposure. We were especially interested in potential impacts of severe egg and larval mortality, because eggs and larvae are particularly vulnerable and cannot actively avoid exposure to toxic substances. Using data on historical abundance of cod from commercial catches and scientific surveys of cod eggs, larvae, juveniles and adults, we constructed a population model to simulate how a hypothetical mortality event that kills a large proportion of larvae would affect the total biomass of the population and its fishery. We found that marine fish species can be remarkably resilient against severe mortality events that affect the survival of a single year-class (all fish born in the same spawning season). Loosing a large proportion of larvae in a given year had a moderate impact on total population biomass and population recovery was relatively fast. Our results further showed that economic damage due to catch losses in subsequent years can be substantial, especially if nearly the entire year-class is lost. However, some potentially important factors were not considered here, including non-lethal effects on the reproductive potential of the population and ecosystem changes such as the collapse of food sources. These factors should be accounted for in future studies in order to gain a more comprehensive appreciation of the ecological and economic implications of catastrophic events such as oil spills in the marine environment.

Publications:

* Ohlberger J, Rogers LA, & Stenseth NC (2014) Stochasticity and determinism: How density-independent and density-dependent processes affect population variability. PLoS ONE 9(6):e98940. 
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0098940

* Ohlberger J, & Langangen Ø (2015). Population resilience to catastrophic mortality events during early life-stages. Ecological Applications 25(5):1348–1356

* Patin R, Rogers L, & Ohlberger J (2015). Using a state-space population model to detect age-dependent species interactions. Canadian Journal of Fisheries and Aquatic Sciences. Accepted for publication.