The ability to recognize individual animals has substantially increased our knowledge of the biology and behaviour of many taxa. However, not all species lend themselves to this approach, either because of insufficient phenotypic variation or because tag attachment is not feasible. The use of genetic markers ('tags') represents a viable alternative to traditional methods of individual recognition, as they are permanent and exist in all individuals. We tested the use of genetic markers as the primary means of identifying individuals in a study of humpback whales in the North Atlantic Ocean. Analysis of six microsatellite loci among 3,060 skin samples collected throughout this ocean allowed the unequivocal identification of individuals. Analysis of 692 'recaptures', identified by their genotype, revealed individual local and migratory movements of up to 10,000 km, limited exchange among summer feeding grounds, and mixing in winter breeding areas, and also allowed the first estimates of animal abundance based solely on genotypic data. Our study demonstrates that genetic tagging is not only feasible, but generates data (for example, on sex) that can be valuable when interpreting the results of tagging experiments.

Although much is known about the humpback whale, Megaptera novaeangliae, regional studies have been unable to answer several questions that are central to the conservation and management of this endangered species. To resolve uncertainties about population size, as well as the spatial and genetic structure of the humpback whale population in the North Atlantic, we conducted a two-year ocean-basin-wide photographic and biopsy study in 1992-1993. Photographic and skin-biopsy sampling was conducted of animals in feeding and breeding areas throughout most of the range of this species in the North Atlantic, from the West Indies breeding grounds through all known feeding areas as far north as arctic Norway. A standardized sampling protocol was designed to maximize sample sizes while attempting to ensure equal probability of sampling, so that estimates of abundance would be as accurate and as precise as possible. During 666 d at sea aboard 28 vessels, 4,207 tail fluke photographs and 2,326 skin biopsies were collected. Molecular analyses of all biopsies included determination of sex, genotype using six microsatellite loci, and mitochondria1 control region sequence. The photographs and microsatellite loci were used to identify 2,998 and 2,015 individual whales, respectively.Previously published results from this study have addressed spatial distribution, migration, and genetic relationships. Here, we present new estimates of total abundance in this ocean using photographic data, as well as overall and sex-specific estimates using biopsy data. We identify several potential sampling biases using only breeding-area samples and report a consistent mark-recapture estimate of oceanwide abundance derived from photographic identification, using both breeding and feeding-area data, of 10,600 (95% confidence interval 9,300-12,100). We also report a comparable, but less Present address: SMITH E T A L . : MARK-RECAPTURE STUDY 3 precise, biopsy-based estimate of 10,400 (95% confidence interval of 8,000-13,600). These estimates are significantly larger and more precise than estimates made for the 1980s, potentially reflecting population growth. In contrast, significantly lower and less consistent estimates were obtained using between-feeding-area or between-breeding-area sampling. Reasons for the lower estimates using the results of sampling in the same areas in subsequent years are discussed. Overall, the results of this ocean-basin-wide study demonstrate that an oceanwide approach to population assessment of baleen whales is practicable and results in a more comprehensive understanding of population abundance and biology than can be gained from smaller-scale efforts.

Beginning in the 1880s, management of marine fisheries by hatching and releasing yolk‐sac‐stage larvae was advocated in both the United States and Norway. Major cod hatchery programs were popular in both countries until the mid‐20th century, despite lack of evidence that cod abundance increases with release of hatchery‐reared fish larvae; the potential value for such management procedures was repeatedly advocated throughout the 20th century. In Norway, a beach‐seine monitoring program was begun in the early 1900s to collect data on fall abundance of 6‐mo‐old demersal fish in 21 fjords along the Norwegian Skagerrak coast and is still going on. We used these data in conjunction with hatchery data on numbers of yolk‐sac larval cod released each spring in several fjords to test for an effect of the releases on the abundance of fjord cod populations. Using both a permutation test and a statistically derived time‐series model for the cod's population dynamics, we found a slight, but statistically significant, dependence of 6‐mo‐old cod abundance on the number of yolk‐sac larvae released in four of the 16 fjords (for which we had adequate release and beach‐seine data needed for carrying out the testing). However, using the time‐series model, we did not find evidence of long‐term increases in the abundance of mature cod in any of the fjords. We discuss our findings on the basis of the literature on marine fish population enhancement programs worldwide.