Harvest selection on Atlantic cod behavioral traits: implications for spatial management

Abstract

Harvesting wild populations may contrast or reinforce natural agents of selection and potentially cause evolutionary changes in life-history traits such as growth and maturation. Harvest selection may also act on behavioral traits, although this field of research has so far received less attention. We used acoustic tags and a network of receivers to monitor the behavior and fate of individual Atlantic cod (Gadus morhua, N = 60) in their natural habitat on the Norwegian Skagerrak coast. Fish with a strong diel vertical migration, alternating between shallow- and deep-water habitats, had a higher risk of being captured in the fishery (traps, gillnet, hand line) as compared to fish that stayed in deeper water. There was also a significant negative correlation between fish size (30-66 cm) and the magnitude of diel vertical migration. Natural selection on behavior was less clear, but tended to favor fish with a large activity space. On a monthly time scale we found significant repeatabilities for cod behavior, meaning that individual characteristics tended to persist and therefore may be termed personality traits. We argue that an evolutionary approach to fisheries management should consider fish behavior. This would be of particular relevance for spatial management actions such as marine reserve design.

Keywords: Atlantic cod; behavior; harvesting; personality trait; selection; telemetry.

Figure 1

Atlantic cod (Gadus morhua). Photograph by Øystein Paulsen used with permission.

Figure 2

Sømskilen study area south off the town of Arendal on the Norwegian Skagerrak coast, showing the network of 25 acoustic monitoring receivers (black dots) used to detect and store signals transmitted from tagged Atlantic cod. Gray lines indicate the 5, 10, and 20 m depth contours.

Figure 3

Positions of individual Atlantic cod during June 2008 estimated from acoustic monitoring. Color denotes logged depth, ranging from 0 m (blue) to 30 m (red). Letters refer to the fate of each individual during July–September 2008: S = survived, H = harvested, D = died, and L = left the area. For map scale and details, see Figure 1.

Figure 4

Box plots of Atlantic cod behavioral traits during June–September 2008, showing vertical position (meter depth), diel vertical migration (meters), horizontal movement pattern (linearity), horizontal movement shifts (SD of linearity), and three-dimensional activity space (m³). All variables are shown on a log scale.

Figure 5

Repeatabilities for individual Atlantic cod behavioral traits across three temporal comparisons (June–July, June–August, and June–September), showing the mean prediction and 95% confidence interval.