No abstract

Prey intake and selection were related to within-shoal position for Atlantic cod (Gadus morhua) engaged in annual migration across the Newfoundland shelf in the northwest Atlantic. Comparisons made among fish occupying five regions from the front to rear of a large (>10 km across) migrating shoal indicated that leading fish, or scouts, were larger, ate more food by weight, and had a more varied diet than did fish at other positions. Also, scouts consumed more preferred prey types (fish and pelagic invertebrates) than did fish at other positions. In contrast, trailing fish consumed few fish prey but a larger proportion of benthic invertebrates. Our results are the first to document systematic heterogeneous feeding success among members of a free-ranging and migrating fish shoal in the open ocean.

The reactions of Atlantic cod (Gadus morhua) in spawning condition to a single pass with an otter trawl were observed by repeatedly transecting the trawl track through a cod shoal with a 38-kHz echosounding system. The shoal consisted of a 5-km-wide band of fish extending approximately 25 km along the 390-m isobath and occupying the bottom 10 m at varying densities averaging 0.004 fish⋅m -3 (maximum 0.488⋅m -3 ). The shoal comprised cod of a mean size of 41 Ϯ 6.1 cm. Following passage of the trawl, a 300-m-wide "hole" in the aggregation spanned the trawl track. Disturbance was detected for 77 min after passage of the trawl. Densities were very low in and near the trawl track and increased up to a distance of 200-400 m on each side of the track (a total distance of 400-800 m). This study is the first to observe large-scale changes in the structure of a shoal of cod in spawning condition, attributable to otter trawling, and indicates that such responses can result in persistent disturbances within the shoal over relatively large distances.Résumé : Pour observer les réactions de morues franches (Gadus morhua) en période de fraye à un seul passage d'un chalut à panneaux, nous avons à plusieurs reprises coupé le sillage du chalut avec un sondeur acoustique de 38 kHz à travers un banc de morues. Le banc était formé d'une mouvée de poissons de 5 km de large, s'étendant sur environ 25 km le long de l'isobathe de 390 m, et occupant une zone de 10 m au-dessus du fond, à des densités variables dont la moyenne se situait à 0,004 poisson⋅m -3 (valeur maximale 0,488 poisson⋅m -3 ). Le banc était formé de morues dont la taille moyenne était de 41 Ϯ 6,1 cm. À la suite du passage du chalut, un « trou » de 300 m de largeur s'est formé dans le groupe de poissons tout au long du sillage laissé par le chalut. Des perturbations ont été détectées pendant 77 min après le passage du chalut. Les densités étaient très basses dans le sillage du chalut et à proximité de celui-ci, et augmentaient sur une distance de 200 à 400 m de chaque côté du sillage (soit une distance totale de 400 à 800 m). Cette étude est la première à rendre compte de changements à grande échelle dans la structure d'un banc de morues en période de fraye, changements qui peuvent être attribués à l'utilisation d'un chalut à panneaux; elle indique que pareilles réactions de la part des poissons peuvent causer des perturbations persistantes au sein du banc sur des distances relativement importantes. [Traduit par la Rédaction]

Acoustic methods used to quantify the shoaling dynamics of Atlantic cod (Gadus morhua) during their shoreward migration across the northeast Newfoundland shelf (spring 1992) showed that shoal structure and horizontal displacement were associated with the density distribution of small pelagic scatterers, for example, shrimp (Pandalus borealis). Cod speeds (to 20 km/d) were lower after encounters with shrimp (4–7 km/d). The relationship between several characteristics of the cod shoal (density, vertical spread, height off the bottom) and shrimp density was dome shaped. Internal shoal densities declined and shoal spread and height increased until shrimp densities surpassed a critical level. Above this threshold, these trends were reversed. The vertical spread of the cod shoal matched that of shrimp up to, but not beyond, a shrimp spread of 85 m. At this observed maximum in the expansion of the shoal, fish were eight body lengths apart. At all times during our survey, cod stayed within the warmer waters (> 2 °C) found at depths greater than 250 m. Our results are the first field verification of theoretical predictions and laboratory findings that shoals expand and contract in response to foraging activity and demonstrate the potential importance of cod aggregation dynamics to interpretations of trawl data.

Data on egg distributions and timing of appearance of maximum egg densities were examined to determine the stock structure of three commercially important flatfish occurring on the Scotian Shelf: American plaice (Hippoglossoides platessoides), yellowtail flounder (Limanda ferruginea), and witch flounder (Glyptocephalus cynoglossus). Distributions of sexually mature females obtained from research vessel surveys were used to further support inferences concerning spawning location. Using such information, more than one stock of both American plaice and yellowtail flounder appear to occur on the Scotian Shelf, but no conclusions could be drawn concerning witch flounder. In the case of American plaice and yellowtail flounder, the existing means of geographic aggregation of data for stock assessment purposes do not appear to adequately represent stock structure. Little or no advection of eggs and larvae appears to occur on the Scotian Shelf, supporting the hypothesis that current-driven retention areas promote stock discreteness in that region. It was concluded that ichthyoplankton surveys can be a valuable adjunct to more traditional means of stock identification.