Spatial Scales of Movement in Northeast Atlantic Coastal Cod

Abstract

The movement patterns of released hatchery-reared fish determine the geographical scale at which a population may be enhanced, while the movement patterns of wild fish affect the management strategies of wild populations. This article investigates (1) if movement patterns differ between hatchery reared and wild cod, and (2) if the movement patterns of coastal cod differ between regions. The results from a large mark-recapture experiment in northern Norway showed that displacement distances (DD) for both wild and hatchery-reared fish were highly skewed. The frequency distribution of DD for wild cod was well described by a log-normal distribution. While reared cod that were released at a small size (< 27 cm in length) and young age (< 1 year of age) had similar movement patterns to wild cod of similar size, reared cod that were larger and older at release dispersed rapidly after release. The frequency of long-distance migrations (DD > 50 km) for larger reared cod was similar, however, to that of wild cod. Fitting log-normal distributions to published mark-recapture data revealed that there was a latitudinal trend, with longer displacement distances in the north than in the south. Cod released in coastal bank areas and offshore island groups also had longer displacement distances than cod released at the Norwegian coast.

Keywords:

• migration patterns
• latitudinal trend
• home ranges
• hatchery reared
• tagging
• Gadus morhua

ACKNOWLEDGMENTS

We thank John Pope for his useful comments about the statistical analyses.

Notes

^A mean length and SD (within brackets) at tagging/release for recaptured fish,

^B 5–7 years of age correspond to 45–75 cm in length (Berg and Albert 2003),

^C range of mean length at tagging of sub-groups,

^D no lengths given, but fish lengths are likely to be > 35 cm at tagging judged from gear,

^E mean length and standard deviation within brackets at tagging/release of tagging groups,

^F range of lengths at tagging. Tag types: Hallprint T-tags and Floy tags are very similar anchor tags. Lea tags are 5 cm long plastic cylinders containing a letter which are fastened to the fish with nylon gut in front of the anterior dorsal fin. DSTs are data storage tags inserted into the body cavity of the fish with an anchoring tail passed outside of the fish. Silver plate tags had a weight of 1 g and were attached to the musculature between the first and second dorsal fin. No. DD gives the number of fish for which displacement distances were estimated. DD method refers to the method used for estimating displacement distances (DD) and for fitting the log-normal distribution. A, DD measured as the shortest possible track; B, log-normal distribution fitted by fixing SD to 1.0 and fitting to the number of fish recaptured within and outside the fjord, assuming all fish were tagged at the head of the fjord; C, DD calculated as the straight line from place of release and point of recapture. Data read from Figure 2 in Svåsand et. al (2000) (upper bin values 5, 10, 50, > 50 km); D, DD measured as straight line displacement. Data was read from bar chart (upper bin values 5, 10, 15, 20, 30, 40, 60, 80, 100, > 100 km); E, calculated sample mean and SD of log(displacement distance); F, numbers given in bins (upper bin values 2, 10, > 10 km); G, numbers given in bins (upper bin values 1, 2, 5, 4, 10, 15, 20, > 20 km); H, read from bar chart (upper bin values 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, > 30 nm); I, DD was measured as shortest possible track from maps in Hylen (1964b).