Dusky dolphin

Abstract: Since 1989, interactions between marine mammals and fisheries have been monitored along the Atlantic Patagonian coast. The Argentinian national fishing fleet in the area is composed of approximately 208 vessels (75% trawlers; 16% jiggins; 9% longliners). The fleet operates the year-round from eight harbours between Escondida Island and San Jorge Gulf (80 000 naut. miles2). The important target species are: hake (Merluccius hubbsi), shortfin squid (IIlex argentinus) and shrimp (Pleoticus muelleri), while several tons of other fish are discarded. The dusky dolphin (Lagenorhynchus obscurus), the Commerson's dolphin (Cephalorhynchus commersonii) and the southern sea lion (Otaria flavescens) are incidentally caught. The dolphins become entangled mainly in mid-water trawls at night, while the sea lions become entangled in any kind of trawl. Annual mortality rates obtained are 170-480 sea lions (mostly males; figure estimated to be 1–2% of the population size in the area), 70–200 dusky dolphins (70% females) and 25–170 Commerson's dolphins. Feeding habits of top predators, fish catch and bycatch of the fishery were also studied. Dusky dolphins in the area prey mainly on anchovy and squid, while hake was the most important food item for Commerson's dolphins and male southern sea lions. Squid is also important prey for sea lions and overlaps in size with the fish taken by the commercial fishery. Fishing areas for squid taken by jigging vessels overlap in part with those of foraging sea lions. Even though the commercial size for hake is 30 cm or larger, smaller hakes are largely discarded by the fishery. A combination of operational and specific interactions have been detected. At present, mortality rates seem to be low and there is insufficient evidence of competition for prey species. However, there is concern about the large amounts of fish (particularly hake) discarded by the fishery.

Abstract: Active-acoustic surveys were used to determine the distribution of dusky dolphins and potential prey in two different New Zealand locations. During seven survey days off Kaikoura Canyon, dusky dolphins were found within the DeepScattering Layer (DSL) at 2000 when it rose to within 125 m of the surface. As the DSL rose to 30 m at 0100, the observed depth of dolphins decreased, presumably as the dolphins followed the vertical migration of their prey. Acoustically identified subgroups of coordinated animals ranged from one to five dolphins. Time, depth of layer, and layer variance contributed significantly to predicting foraging dusky dolphin subgroup size. In the much shallower and more enclosed Admiralty Bay, dolphins noted at the surface as foraging were always detected with the sonar, but were never observed in coordinated subgroups during the brief (two-day) study there. In Admiralty Bay dolphin abundance was correlated with mean volume scattering from potential prey in the water column; and when volume scattering, an index of prey density, was low, dolphins were rarely present. Ecological differences between the deep waters of Kaikoura Canyon and the shallow nearshore waters of Admiralty Bay may result in differences in how, when, and in what social groupings dusky dolphins forage.

Abstract: Marine farms have the potential to affect dolphin foraging in the coastal environment, yet this issue has been largely omitted from aquaculture management models. Data on the subject are minimal. This study was conducted to examine potential overlap between dusky dolphin habitat use and New Zealand's growing green-lipped mussel farming industry. Data on dusky dolphin occurrence, distribution, abundance, and behaviour were collected from small vessels over five successive winters in the Marlborough Sounds, the centre of New Zealand's mussel farming industry. Locations and movements of dolphin groups were recorded at 2 min intervals with a global positioning system receiver to examine the overlap of dusky dolphin use of coastal areas with existing and proposed marine farms. All cases of dolphins entering the boundaries of mussel farms and total time spent in farms were recorded. Over 8500 dolphin dorsal fin photographs were analysed to develop a catalogue of 421 marked individuals utilizing the area. All instances of dolphin feeding were noted, and focal group behaviour was recorded at 2 min intervals for groups observed ⩾1 h. Within the Marlborough Sounds, dusky dolphins were most often encountered during the winter in Admiralty Bay, the area with the greatest density of proposed farming activity in the region. Mark-recapture data indicate that more than 1000 dusky dolphins used Admiralty Bay over the course of the 5 year study, with an average of 220 individuals inhabiting the bay on any given week during the winters of 1998–2002. As many as 55% of individuals returned to Admiralty Bay in consecutive winters. Overlap of dusky dolphin habitat use with proposed marine farms is high, and dolphins rarely used areas within the existing farms. If dusky dolphin distribution with respect to farms were random, an expected 18plusmn1;0.5 of 436 groups would be encountered in existing inner Admiralty Bay farms; however, no dolphin groups were first encountered in farms. In 5 years, only eight of 621 dusky dolphin groups monitored in Admiralty Bay were observed to enter the boundaries of mussel farms at any point. Dolphins entering mussel farms moved rapidly up the lanes between rows of lines and floats. Dolphins were observed a total of 14.2 min inside farms versus 147.5 h outside of farms in Admiralty Bay. Correcting for area, dolphins were observed spending significantly less time per survey inside than outside of farms. Most dusky dolphin groups in Admiralty Bay were observed feeding on small schooling fish, often associated with seabirds and/or fur seals. Movement and diving patterns indicate much foraging when not actively feeding. Regular seasonal migration of dusky dolphins and frequent feeding associations with other apex predators make management of marine farming a wider socio-economic and ecological issue. Copyright © 2004 John Wiley & Sons, Ltd.

Abstract: We investigated the phylogeography and evolutionary history of dusky dolphins (Lagenorhynchus obscurus) using DNA sequences of the full mitochondrial cytochrome b gene in 124 individuals from the putative stocks off Peru, Argentina and Southwest Africa While genetic differentiation within oceans is surprisingly low, there is no evidence for recent female gene flow between Atlantic and Pacific waters Highest genetic variability in terms of sequence divergence and number of haplotypes is found in the Atlantic Our analyses also indicate that the eastern South Pacific dusky dolphins stock should be considered a separate management unit Given the high level of mortality experienced by the Peruvian dusky dolphin in local fishery activities, these findings have important implications for an objective management of the species Furthermore, we analysed our mitochondrial sequence data with several widely used network estimation and rooting methods The resulting intraspecific gene genealogies and rooting inferences exhibited substantial differences, underlying the limitations of some algorithms Given that scientific hypotheses and management decisions depend strongly on inferred tree or network topologies, there is a clear need for a systematic comparative analysis of available methods Finally, the present study indicates that (i) the dusky and the Pacific white-sided dolphins are sister species and (ii) not only the Westwind Drift hypothesis but also other models of dispersion are compatible with the current geographical distribution of dusky dolphins