Silvana L. Dans

INCIDENTAL CATCH OF DOLPHINS IN TRAWLING FISHERIES OFF PATAGONIA, ARGENTINA: CAN POPULATIONS PERSIST?

The incidental mortality of dusky dolphins (Lagenorhynchus obscurus) in the Patagonian trawling fishery is a well-known problem, especially in mid-water trawl fisheries for shrimp and anchovy. Therefore, it is necessary to determine if recent levels of dolphin bycatch are compatible with the persistence of the dusky dolphin population. This was assessed by comparing annual bycatch rates with a sequence of critical values. These values were based on the maximum rate of increase (rmax) = λ − 1, where λ is the finite rate of increase. The finite rate of increase was estimated from a Leslie matrix where Monte Carlo procedures were employed to incorporate parameter uncertainties (age at first reproduction, survival rates, and fertility). Model life tables were used for survival, and reproductive parameters were estimated from available data on calving interval and age at sexual maturity. Uncertainties associated with annual bycatch rates were also included in the analysis through another randomization procedure that incorporated the variability in dolphin catch per unit effort, nominal fishing effort, and population size. This modeling approach rendered frequency distributions for the parameters of interest. Considering the critical values for incidental mortality rate as (1/2)R = (1/2)rmax and (1/4)R = (1/4)rmax, and a 2-yr calving interval scenario, the probability that recent dolphin bycatch rates exceed the maximum annual removal rates were higher than 0.3. These results suggest that incidental mortality due to fishing could be, or may become, a threat for the dusky dolphin population off Patagonia. Therefore, a more accurate and intense monitoring program is needed to avoid the decline of this population. Also, this issue should be included in the fishery management strategies to be implemented in the near future.

Social Composition and Spatial Distribution of Colonies in an Expanding Population of South American Sea Lions

Abstract In northern Patagonia, South American sea lions (Otaria flavescens) are increasing in number after a depletive harvest. There has been an expansion of colonies with an increase in numbers of pups, and changes in social composition and spatial distribution. Using annual counts of 4 different age classes from 1972 to 2007, we found that new colonies change their social composition, transforming from haul-out sites to breeding sites while passing through mixed structures. From this pattern, we hypothesize that at dense breeding sites the shortage of space or the avoidance of intraspecific harassment promotes dispersal by 1st-time breeders to suitable sites nearby. Such mechanisms, along with philopatry and site fidelity, will promote the establishment of new breeding colonies nearer to existing breeding colonies than would be expected by chance. There was significant spatial clustering of new breeding colonies around the 7 focal (established) colonies. This spatial pattern was consistent through time. New breeding colonies were closer to focal colonies than are nonbreeding ones, suggesting a “spill-over” effect, where young individuals choose to breed near established breeders. The colonization mechanism we found suggests that potential areas for population expansion could be closer to areas where growing colonies already exist and highlights the importance of the juvenile age classes and the areas adjacent to colonies in the overall recovery of any population of pinnipeds.

Diurnal Behavior of Dusky Dolphins, Lagenorhynchus obscurus, in Golfo Nuevo, Argentina

Abstract Standardized measures of behavior can be powerful tools for predicting effects of human activities on natural populations of mammals. We quantified the diurnal activity budget of dusky dolphins (Lagenorhynchus obscurus) in Golfo Nuevo, Argentina, by examining variation in activity as a function of season and age composition of social groups. Observations were made from a research vessel during summer and autumn from 2001 to 2005. Focal group-follow methodology was used. The predominant activity in each social group was recorded using instantaneous sampling, with a 2-min interscan interval. The main daytime activity of dusky dolphins was traveling, followed by milling and feeding. Mother and calf pairs spent more time milling and resting, whereas larger groups of adults and juveniles as well as mixed–age-class groups spent more time traveling and feeding. Although a seasonal pattern of variation in group size and composition was found, little seasonal variation was found in activity budgets, which were almost constant during daylight hours. The activity budget generated by this study provides a baseline for detection of behavioral differences associated with tourism and other human activity in the region.

Effects of Tour Boats on Dolphin Activity Examined with Sensitivity Analysis of Markov Chains

In Patagonia, Argentina, watching dolphins, especially dusky dolphins (Lagenorhynchus obscurus), is a new tourist activity. Feeding time decreases and time to return to feeding after feeding is abandoned and time it takes a group of dolphins to feed increase in the presence of boats. Such effects on feeding behavior may exert energetic costs on dolphins and thus reduce an individuals survival and reproductive capacity or maybe associated with shifts in distribution. We sought to predict which behavioral changes modify the activity pattern of dolphins the most. We modeled behavioral sequences of dusky dolphins with Markov chains. We calculated transition probabilities from one activity to another and arranged them in a stochastic matrix model. The proportion of time dolphins dedicated to a given activity (activity budget) and the time it took a dolphin to resume that activity after it had been abandoned (recurrence time) were calculated. We used a sensitivity analysis of Markov chains to calculate the sensitivity of the time budget and the activity-resumption time to changes in behavioral transition probabilities. Feeding-time budget was most sensitive to changes in the probability of dolphins switching from traveling to feeding behavior and of maintaining feeding behavior. Thus, an increase in these probabilities would be associated with the largest reduction in the time dedicated to feeding. A reduction in the probability of changing from traveling to feeding would also be associated with the largest increases in the time it takes dolphins to resume feeding. To approach dolphins when they are traveling would not affect behavior less because presence of the boat may keep dolphins from returning to feeding. Our results may help operators of dolphin-watching vessels minimize negative effects on dolphins.

Dusky dolphin: modeling habitat selection

Abstract The use of habitat selection models to predict the likely occurrence of wild populations is an important tool in conservation planning and wildlife management. The goal of our study was to build habitat selection models for the dusky dolphin (Lagenorhynchus obscurus) in Golfo Nuevo, Argentina. Random transects were surveyed by boat in 2002–2007. A grid of 1.5 × 1.5-km squares (cells) was constructed for the study area. We characterized each cell by depth, slope, distance from shore, sea-surface temperature, concentration of chlorophyll a, presence–absence of dolphins, and a coefficient of use by dusky dolphins. Models were developed for warm and cold seasons and for all data combined. Data collected during 2002–2005 were pooled to develop the model, and data collected during 2006–2007 were used for cross-validation. Logistic regression with a binomial error structure and a logit-link function were used to relate the presence of dolphins to habitat variables. Models with gamma structure and log-li...

Solapamiento trófico entre el lobo marino de un pelo Otaria flavescens y la pesquería de arrastre demersal del golfo San Matías, Patagonia, Argentina

As world fisheries began to decline and massive collapses were observed, the competition between marine mammals and fisheries became an issue of growing concern. San Matias Gulf (Patagonia, Argentina) is considered to be a fishery ecosystem independent of the Argentine Continental Shelf waters, with particular oceanographic and biological properties. As a semi-enclosed ecosystem, this gulf may generate particular scenarios for interactions between the demersal trawl fishery fleet and the population of South American sea lions Otaria flavescens. In this paper, the diet of the top predator and the composition of fishery catches were characterized in order to assess the trophic overlap between these two components. This analysis and a comparison of the sizes of prey consumed revealed a low probability of competition for similar resources between the fishing fleet and the marine mammals in the San Matias Gulf ecosystem.

A hunted population in recovery: Effective population size for South American sea lions from Patagonia

Effective population size ( N e ) is a parameter of central importance in evolutionary biology and conservation. Factors such as unequal sex ratios of breeding individuals, periodic fluctuation in population size and variance in reproductive success can affect the N e in general. At present, South American sea lions, Otaria flavescens, from northern Patagonia, Argentina, belongs to one of the several populations that are recovering from overhunting which occurred in the early 20th century. Here, we present the estimate of N e for this population that takes into account the effects of their polygynous mating system and variation in population size through time. The resultant overall N e ’s were 4171 ± 2450 or 4745 ± 2681 breeding animals depending on the inclusion of peripheral adult males. The estimated N e ’s are not critical, because they are close to the average mean minimum viable population for vertebrates (5000 breeding adults). Even though the northern Patagonian population of O. flavescens is increasing its abundance is still far below its historical numbers. The N e estimated should be considered the minimum range to be maintained, especially in light of bycatch related to fishery interactions along the Patagonian coast. We emphasise the importance of estimating N e for future management plans of the species within different populations.

Diving for food: a switch of foraging strategy of dusky dolphins in Argentina

During winter, dusky dolphins (Lagenorhynchus obscurus) were observed in coordinated diving apparently in a feeding activity, contrasting with the surface feeding observed during summer. The aim of this work consisted in analyzing the diving activity as an alternative foraging strategy in Argentine dusky dolphins, based on sequential analysis. The study area was Golfo Nuevo, located in Northern Patagonia, Argentina. Random transects were surveyed by a research boat from 2001 to 2007. During behavioral sampling, group members were observed continuously and the predominant activity was recorded at 2-min intervals. Six predominant activities were identified. Each 2-min interval was classified according to the activity at the previous interval (preceding activity), the activity at the interval (following activity), and the season (cold or warm). Z scores were calculated and then used to construct sequence diagrams. An association between diving and milling behavior was observed. This could be another foraging tactics different to the surface foraging sequences and this could be related with the distribution or abundance of preys.

Gastrointestinal parasites of bottlenose dolphins Tursiops truncatus from the extreme Southwestern Atlantic, with notes on diet composition.

We surveyed the gastrointestinal tracts of 6 bottlenose dolphins Tursiops truncatus from Patagonia to check for helminth parasites and characterize dolphin diet. All dolphins harbored parasites (6477 helminths). We recorded 7 species, including nematodes Anisakis simplex s.l., Pseudoterranova decipiens, acanthocephalans Corynosoma cetaceum, C. australe, and digeneans Braunina cordiformis, Pholeter gastrophilus and Synthesium tursionis. Among the gastric helminths, the most prevalent species were C. cetaceum and A. simplex while C. australe and S. tursionis inhabited the intestine at low prevalence. This is the first report of C. australe and P. decipiens in bottlenose dolphins. Regarding diet, 5 stomachs contained food remains (consisting of 103 prey items). The most important prey species were Geotria australis and Stromateus brasiliensis, but their role in parasite transmission is unclear. At the community level, the gastrointestinal parasite community of T. truncatus was depauperate and strongly overlapped the community described for pelagic dolphins inhabiting Patagonia, suggesting a strong local influence in shaping helminth communities. Nevertheless, these observations are at odds with the notion that oceanic cetaceans have comparatively poorer helminth fauna than neritic species such as bottlenose dolphins, due to the lower likelihood of parasite exchange.

Seabird and dolphin associations: do seabirds benefit from feeding in association with dusky dolphins in Patagonia?

The objective of this study was to describe associations between several species of seabirds and dusky dolphins. We investigated during what dolphin activities seabirds were most commonly associated, and the size of flock in relation to the number of dolphins in a group. Since both seabirds and dolphins may display different feeding strategies, we also investigated if benefits differed among seabird species. Data were collected in Golfo Nuevo (42°20′S65°00′W) on-board a research vessel between 2001 and 2008. A total of 224 mixed groups of seabirds were encountered during this study. The seabird–dolphin associations were mainly observed during dusky dolphin surface feeding. Shearwaters and kelp gulls were mainly observed in flocks that were associated with dolphins, while Magellanic penguins and cormorants were mainly observed without dolphins. Seabirds may be conditioned to the foraging strategy of dolphins, since birds are associated with dolphins only during dolphin surface feeding. This association probably helped seabirds to find prey, but there were no obvious benefits to dolphins.