Populations of northeastern offshore spotted dolphins Stenella attenuata attenuata and eastern spinner dolphins S. longirostris orientalis have been reduced because the dolphins are bycatch in the purse-seine fishery for yellowfin tuna in the eastern tropical Pacific Ocean (the 'tuna-dolphin issue'). Abundance and trends of these dolphin stocks were assessed from 12 large-scale pelagic surveys carried out between 1979 and 2000. Estimates of abundance were based on a multivariate line-transect analysis, using covariates to model the detection process and group size. Current estimates of abundance are about 640 000 northeastern offshore spotted dolphins (CV = 0.17) and 450 000 eastern spinner dolphins (CV = 0.23). For the whole period from 1979 to 2000, annual estimates of abundance ranged from 494 000 to 954 000 for northeastern offshore spotted dolphins and from 271000 to 734 000 for eastern spinner dolphins. Management actions by USA and international fishing agencies over 3 decades have successfully reduced dolphin bycatch by 2 orders of magnitude, yet neither stock is showing clear signs of recovery. Possible reasons include underreporting of dolphin bycatch, effects of chase and encirclement on dolphin survival and reproduction, long-term changes in the ecosystem, and effects of other species on spotted and spinner dolphin population dynamics.

/pdf/non-recovery-of-two-spotted-and-spinner-dolphin-populations-5cf63l4wuo.pdf

Non-recovery of two spotted and spinner dolphin populations in the eastern tropical Pacific Ocean

Drafting in cetaceans is defined as the transfer of forces between individuals without actual physical contact between them. This behavior has long been surmised to explain how young dolphin calves keep up with their rapidly moving mothers. It has recently been observed that a significant number of calves become permanently separated from their mothers during chases by tuna vessels. A study of the hydrodynamics of drafting, initiated in the hope of understanding the mechanisms causing the separation of mothers and calves during fishing-related activities, is reported here. Quantitative results are shown for the forces and moments around a pair of unequally sized dolphin-like slender bodies. These include two major effects. First, the so-called Bernoulli suction, which stems from the fact that the local pressure drops in areas of high speed, results in an attractive force between mother and calf. Second is the displacement effect, in which the motion of the mother causes the water in front to move forwards and radially outwards, and water behind the body to move forwards to replace the animal's mass. Thus, the calf can gain a 'free ride' in the forward-moving areas. Utilizing these effects, the neonate can gain up to 90% of the thrust needed to move alongside the mother at speeds of up to 2.4 m/sec. A comparison with observations of eastern spinner dolphins (Stenella longirostris) is presented, showing savings of up to 60% in the thrust that calves require if they are to keep up with their mothers. A theoretical analysis, backed by observations of free-swimming dolphin schools, indicates that hydrodynamic interactions with mothers play an important role in enabling dolphin calves to keep up with rapidly moving adult school members.

/pdf/the-hydrodynamics-of-dolphin-drafting-483zm8hjij.pdf

The hydrodynamics of dolphin drafting

Many severely depleted populations of baleen whales (Mysticeti) have exhibited clear signs of recovery whereas there are few examples in toothed whales (Odontoceti). We hypothesize that this difference is due, at least in part, to social and behavioural factors. Clearly, a part of the lack of resilience to exploitation is explained by odontocete life history. However, an additional factor may be the highly social nature of many odontocetes in which survival and reproductive success may depend on: (a) social cohesion and organization, (b) mutual defence against predators and possible alloparental care, (c) inter-generational transfer of “knowledge”, and (d) leadership by older individuals. We found little evidence of strong recovery in any of the depleted populations examined. Their relatively low potential rates of increase mean that odontocete populations can be over-exploited with take rates of only a few percent per year. Exploitation can have effects beyond the dynamics of individual removals. Four species showed evidence of a decrease in birth rates following exploitation; potential mechanisms include a deficit of adult females, a deficit of adult males, and disruption of mating systems. The evidence for a lack of strong recovery in heavily exploited odontocete populations indicates that management should be more precautionary.

https://downloads.hindawi.com/journals/jmb/2012/567276.pdf

Social and Behavioural Factors in Cetacean Responses to Overexploitation: Are Odontocetes Less “Resilient” Than Mysticetes?

/pdf/interactions-between-cetaceans-and-fisheries-in-the-5eiq1imc4w.pdf

Interactions between Cetaceans and Fisheries in the Mediterranean Sea

We characterize the life-history strategy of female short-beaked common dolphins (Delphinus delphis L., 1758) inhabiting the eastern tropical Pacific by estimating several growth and reproductive p...

Growth and reproduction of female short-beaked common dolphins, Delphinus delphis, in the eastern tropical Pacific

The kill of dolphins (Stenella attenuata and S. longirostris) in the eastern tropical Pacific tuna purse-seine fishery has been underestimated because of unobserved deaths of nursing calves due to separation from their mothers during fishing. Based on an analysis of dolphins killed from 1973 to 1990, and depending on the length at which calves are assumed to become independent, there was a deficit of calves relative to the number of lactating females killed in 24%-32% of 1,847 spotted-dolphin sets and in 13%-19% of 563 spinner-dolphin sets. We found a deficit of 0.31-0.45 spotted dolphin and 0.15-0.26 spinner dolphin calves per set. If these missing calves were added to the observed kill, it would represent an increase in the kill of 10%-15% for spotted dolphins and 6%-10% for spinner dolphins in the sets we examined. We did not attempt to estimate the actual number of unobserved calf deaths due to purse-seine fishing on dolphins, either in the sets we examined or in all dolphin sets. The actual number of unobserved calf deaths is likely to be higher than the calf deficit we found. Separation of dolphin mothers from calves could occur at any of several points in the fishing process, but most of these would be invisible to us and not produce a calf deficit. Estimation of the actual number of unobserved calf deaths would require further information on how frequently permanent mother-calf separations occur, the fraction of calves that survive after separation, the fraction of lactating females set upon that are carrying calves, and the fraction of calves killed that are actually related to lactating females killed in the same set. In any case, the observation of a calf deficit indicates that the reported dolphin kill fails to measure the full impact of purse-seine fishing on spotted and spinner dolphin populations.

Unobserved kill of nursing dolphin calves in a tuna purse‐seine fishery

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Unobserved kill of nursing dolphin calves in a tuna purse‐seine fishery