Richard E. Crawford

Retention of winter flounder larvae within a Rhode Island salt pond

Two winter flounder, Pseudopleuronectes americanus, spawning sites in a 630 hectare Rhode Island lagoon were located by collecting eggs with a modified epibenthic sled towed by boat. A two-dimensional vertically-averaged hydrodynamic model predicted that larvae hatched at these spawning sites would be minimally displaced by tidal movement. Ichthyoplankton samples taken hourly during the day at six locations on March 27 and April 27, 1981 reflected the larval distribution predicted by the model. Larval retention within this lagoon appeared to be strongly influenced by the hydrodynamics of this system. It is suggested that the hydrodynamic features of lagoons are exploited in the reproductive strategies of estuarine species and that the relationship between hydrodynamics and the quality of nursery habitat must be considered before making hydraulic modifications to these systems.

Summer foraging behaviour of shallow-diving seabirds and distribution of their prey, Arctic cod ( Boreogadus saida ), in the Canadian Arctic

Productive areas in the Canadian Arctic seasonally provide top predators with accessible and often predictable sources of energy. Arctic cod (Boreogadus saida) aggregate in shallow bays during the summer and are exploited by seabirds and marine mammals. Information concerning how prey is presented to predatory seabirds, and the cues seabirds use to optimize foraging potential, is limited. Hydroacoustic surveys were completed in Allen Bay, Nunavut, to determine the presence, density, abundance, and depth of Arctic cod schools in relation to shallow-diving seabirds. Schools were also documented using standardized protocols to examine the influence of environmental variables, such as wind, ice, tidal states and seabird behaviour. The presence of schools was a significant predictor of the distribution of northern fulmars (Fulmarus glacialis) but not black-legged kittiwakes (Rissa tridactyla). Glaucous gulls (Larus hyperboreus) associated with northern fulmars are likely optimizing chances of stealing Arctic cod. The density, size and depth of schools did not significantly affect the distribution of the seabirds. We speculate that Arctic cod from demersal schools separate to feed at the surface in satellite schools (groups of dispersed fish), thus reducing competition but increasing the risk of predation.

Observation of common raven (Corvus corax) scavenging Arctic cod (Boreogadus saida) from seabirds in the Canadian High Arctic

The common raven (Corvus corax) is one of the most intelligent avian species, known for its ability to scavenge from humans and other animals. This adaptive nature is critical in habitats where food can be scarce. The Arctic is such an environment, and optimizing associations with sources of prey is important. Large aggregations of Arctic cod (Boreogadus saida) in shallow waters provide a high energetic source for top predators such as seabirds and marine mammals, and consequently potential food sources for ravens. We document, to our knowledge, the first observations of a raven feeding on Arctic cod discarded by seabirds. This report supplements knowledge of ravens to opportunistically meet dietary requirements and the importance of Arctic cod to avian ecology in the Arctic.

Occurrence of a gelatinous predator (Cyanea capillata) may affect the distribution of Boreogadus saida, a key Arctic prey fish species

Although the Arctic cod (Boreogadus saida) has a pan-Arctic distribution, little is known about its occurrence in near-shore waters where this species is the principal prey for seabirds, marine mammals and other fish. Published research describes the scyphomedusa Cyanea capillata as an Arctic cod predator, and this paper presents observations from long-term investigations using active hydroacoustics that suggest the Arctic cod avoided C. capillata in two small bays of Cornwallis Island (Canadian High Arctic archipelago). Distribution patterns in echograms suggested that features such as boundary layer fronts restricted jellyfish movements and Arctic cod were often abundant on the side of fronts where C. capillata were absent. Thus, habitat partitioning allowed Arctic cod to share habitat with its predator, albeit exceptions to this sharing occurred when jellyfish abundance was high and Arctic cod were displaced. Thus, if a warmer Arctic triggers an increase in C. capillata abundance, it is possible that small-scale aspects of Arctic cod distribution could be affected. This in turn could have significant ripple effects within the Arctic food web, an additional and previously unrecognized consequence of climate change.