Increasing density of juvenile Atlantic ( Gadus morhua) and Greenland cod ( G. ogac) in association with spatial expansion and recovery of eelgrass ( Zostera marina) in a coastal nursery habitat

Abstract

Global declines of eelgrass ( Zostera marina) have major ramifications for the juvenile fishes for which this plant can provide nursery habitat. However, it is less clear how rapidly fishes can recover when habitat loss is reversed, either through natural growth or through habitat enhancement efforts. We investigated the consequences of natural expansion and recovery of eelgrass following disturbance, on the densities of juveniles of two cod species – Atlantic ( Gadus morhua) and Greenland ( Gadus ogac) cod – in a coastal nursery area in Newfoundland, Canada. Eelgrass has been expanding at this location, likely as a result of reduced disturbance from winter pack-ice in the past decade. We conducted bi-weekly sampling of juvenile fish densities at 12 sites from July to November annually, 1995–2006, by seine netting. Each site was designated as either ‘natural’, ‘expansion’, ‘removal–recovery’ or ‘unvegetated’ dependent on their recent eelgrass coverage and involvement in a manipulation study (1999–2000; Laurel et al. 2003b). We measured eelgrass percentage cover at each site by analysis of aerial photographs. Eelgrass cover had noticeably increased in the 1999–2000 ‘removal–recovery’ sites and at the ‘expansion’ sites following 2000. Using randomized intervention analysis (RIA) we detected positive relationships between eelgrass cover and relative fish density during 2001–2006. These results suggested a significant and rapid increase in age-0 cod density associated with eelgrass habitat expansion. We suggest that juvenile cod density responds to annual expansion and contraction of eelgrass cover, illustrating the potential cascading effects of eelgrass loss associated with climate change, human disturbance, and disease and the potential benefits of eelgrass conservation and enhancement.