No abstract

The diel feeding periodicity, daily ration and prey selection of juvenile chinook salmon, Oncorhynchus uhawyrscha, were studied in relation to the available prey. Maximum dry weight of food intake occurred about dawn, when mayflies were the major prey, but the greatest number of freshly eaten prey occurred during the afternoon, when chironomids and terrestrial dipterans predominated. Feeding activity at night was low, with smaller mayflies comprising up to 50% of the prey. During the day the young salmon fed selectively onchironomids and the larger mayflies, while trichopterans and terrestrial taxa were under-represented in the diet. Food consumption over the 24-h period averaged 8.3% of the fish dry body weight. Prey abundance in the drift explained about 50% of the composition of the diet. Although the fish selected larger mayflies, sizeapparently was not a main criterion for selection because chironomids, although smaller than mayflies, were also frequently eaten. Previous dietary experience of the fish and the diel pattern of prey abundance appear to best explain the selective feeding ofjuvenile chinook salmon.

Incidental bycatch of seabirds in commercial fishing activities is known to cause declines in seabird populations. However, the full impacts on the ecology of seabirds, including effects on seabird distribution and behavior, through the association with fisheries are not fully understood. We developed a novel method to integrate fine-scale GPS tracking data from the foraging trips of 25 white-capped albatross Thalassarche steadi within sub-Antarctic New Zealand with fishing effort distribution data to (1) quantify fine-scale overlap between individual albatrosses and individual vessels and (2) characterize behavioral changes in albatrosses when they are associated with fishing vessels. Overlap between vessels and albatrosses occurred in 68% of tracks. However, albatrosses demonstrated high variability in foraging trip destinations and association rates with fishing activity, both between and within individuals. Eight tracks never overlapped a fishing vessel. Of the 17 tracks that did overlap, a range of 2 to 73% of foraging effort on each trip occurred while overlapping a fishing vessel. Albatross foraging behavior was characterized by a significantly slower and straighter path when overlapping vessels. This study highlights the utility of GPS tags to examine the fine-scale distribution of seabirds in relation to fishing activity, revealing how effects of fisheries on marine megafauna may extend beyond mortality and injury as well as population numbers. However, results are currently constrained, not only by limited tracking data sets, but also by the quality (spatio-temporal resolution) and availability of fishing effort data. Critical conservation issues related to the effects of fisheries on threatened marine megafauna cannot be fully addressed without comparative data sets with resolution equal to GPS tags.

Benthic and drifting invertebrates as well as fish were sampled in a lake-inlet stream over a 24-h period in summer in order to investigate interactions for food and space between a native fish species (Galaxias brevipinnis, Galaxiidae) and juveniles of two exotic fish species (Salmo trutta and Oncorhynchus mykiss, Salmonidae) in New Zealand. Galaxias brevipinnis fed almost exclusively at night on the benthos, whereas both trout species fed mostly during the day on drifting and benthic prey. Larvae of Deleatidium (Ephemeroptera), Hydrobiosis and Oxyethira (Trichoptera), and Austro-simulium and Chironomidae (Diptera) were the major prey in the diets of all three fish species. Small prey made up a greater proportion of the diet of G. brevipinnis than for either trout species. All three species of fish were found primarily in riffles and runs in water depths of 0.1-0.3 rn, with G. brevipinnis frequently being in faster-flowing areas (water velocity 0.8-1.1 m s-1) than was either trout species (water velocity 0.4-0.7 rn s-1). These differences in pattern of die1 feeding and micro- habitat use no doubt lessen the interaction between the galaxiid and salrnonid fish species.

Recolonisation of previously dry channels by stream invertebrates was studied in the Rakaia River during winter 1981 and summer 1982. The winter experiment continued for 42 days, with stable low flows, whereas the summer experiment was characterised by fluctuating large flows which caused it to be abandoned after 27 days. The fauna was dominated numerically by Chironomidae, a leptophlebiid mayfly (Deleatidium), and oligochaetes during the winter, and by Deleatidium alone during the summer. Recolonisation was considered complete after 33 days in winter and 15 days in summer. Flow fluctuations were the main factor affecting colonisation rates, and it was assumed that drift was the main source of colonising animals. Small freshes during low-flow periods in winter resulted in a rapid increase in total density of invertebrates and number of taxa present and also affected the population structure of Deleatidium larvae in colonisation baskets. Before these freshes numbers had increased steadily over a 27 day period. In summer large floods during high flow periods initially decreased benthic invertebrate numbers in samples but numbers increased rapidly once the flood had passed. This appears to be the first study of its kind on a large unstable river system.

Foraging strategies of southern Buller's albatrosses Diomedea b. bulleri breeding on The Snares, New Zealand, were investigated by satellite telemetry in 1995-97 (10M 10F, 1-24 foraging trips per bird, total of 152 trips). Four sequential patterns were identified, with abrupt changes after hatching, at the end of the guard-stage, and in midJune irrespective of chick development. Birds made long trips to the Tasman Sea (mean duration 12.8 d, mean range 1484 km) or east coast of the South Island (ECSI; 12.3 d, 761 km) during incubation, mostly short trips (1.3 d, 199 km) east of The Snares during the guard-stage. From the end of the guard-stage to mid-June, all birds alternated between 1-2 long trips to the ECSI (5.6 d, 776 km), and 1-4 short trips mostly east of The Snares (1.2 d, 186 km). After mid-June, males reverted to short trips only (2.6 d 270 km until late June, 2.0 d 176 km in July), whereas one female allocated nearly all foraging time to long trips and switched to the west coast of the South Island (4.4 d, 647 km). When adopting the same strategy as males, females tended to forage in different sectors (incubation and guard-stage), further afield (long trips to the ECSI) or at intermediate distances (short trips during the post-guard stage), and/or over deeper waters (long trips, short trips during the guard-stage). Sexual differences in foraging range (but not water depth frequentation) were consistent with male dominance. Results of this and other recent studies highlight the complexity of foraging decisions by breeding seabirds, and the potential for biases when pooling data series from different months or sexes.

No abstract

To determine the diel feeding periodicity and diet of a lake-inlet population of Galaxias brevipinnis in New Zealand, samples of benthos, drift and fish were collected over a 24-h period in summer. Both numerically and gravimetrically, fish stomach contents revealed that feeding started some time after sunset, peaked towards midnight, and virtually ceased after sunrise. Benthic invertebrates were consumed almost exclusively, with ephemeropterans, trichopterans, and dipterans constituting 95% of the total foods eaten, a major proportion being chironomid larvae. Size of prey eaten compared with size of invertebrates in the benthos differed significantly during the night; this indicated that some size-selective feeding occurred. The benthic feeding habit of Galaxias brevipinnis appears to allow it to forage on relatively small prey during the night, a phenomenon that has been reported for other bottom-dwelling native fish species in New Zealand.