Stephen M. Dawson

Pingers, porpoises and power: Uncertainties with using pingers to reduce bycatch of small cetaceans

Incidental mortality in gillnets is probably the most serious global threat to dolphin and porpoise populations. In 1994, a well-designed study demonstrated a 92% reduction in bycatch of harbour porpoises in sink gillnets equipped with acoustic pingers. This result has not yet been fully replicated; in the New Hampshire area where the experiment was conducted or elsewhere. Statistical power analyses indicate that such studies are feasible only in areas of high entanglement rate. Currently unanswered research questions include whether the 1994 results can be replicated, whether habituation might decrease effectiveness over time, and what the mechanism of deterrence is. Practical constraints include the size, cost and battery life of current pingers, and whether their use could be monitored cost-effectively. From a management perspective, even if the effectiveness of pingers is confirmed, widespread incorporation of them into gillnets may not alone be sufficient to meet the requirements of the US Marine Mammal Protection Act. For this reason scientists, managers and fishers must continue to explore other options, including time/area closures and encouragement of more selective fishing methods.

Site fidelity and along-shore range in Hector's dolphin, an endangered marine dolphin from New Zealand

Abstract To document site fidelity and the alongshore range of individual Hectors dolphins we analysed sightings of 32 photographically identified dolphins, each seen ⩾10 times at Banks Peninsula, New Zealand, between 1985 and 1997. The furthest two sightings of an individual were 106 km apart. All other individuals ranged over less than 60 km ( x =31.0 km, SE=2.43) of coastline. Gender did not significantly influence alongshore range (female x =30.4 km, SE =3.21, n =18; male x =27.4 km, SE=5.68, n =5). Site fidelity was high: for example, on average, individuals were seen in Akaroa Harbour for about two thirds of the years they were known to be alive. These data suggest that impacts on Hectors dolphins are most appropriately managed on a small spatial scale.

Bycatch of yellow-eyed penguins (Megadyptes antipodes) in gillnets in New Zealand waters 1979–1997

Between 1979 and 1997, autopsies of 185 yellow-eyed penguins were conducted as part of a long-term study of their population biology. Twenty-one penguins known to have been drowned in gillnets were used to compile a set of physical features and injuries characteristic of death by gillnet entanglement. A further 21 birds whose cause of death was initially unknown showed features consistent with death by gillnet entanglement. Fishers reported a further 30 gillnet entanglements to government agencies. Considering the rarity of this species on the South Island of New Zealand, gillnet entanglement is interpreted to be a significant threat to South Island populations.

Measuring sperm whales from their clicks: stability of interpulse intervals and validation that they indicate whale length.

Multiple pulses can often be distinguished in the clicks of sperm whales (Physeter macrocephalus). Norris and Harvey [in Animal Orientation and Navigation, NASA SP-262 (1972), pp. 397-417] proposed that this results from reflections within the head, and thus that interpulse interval (IPI) is an indicator of head length, and by extrapolation, total length. For this idea to hold, IPIs must be stable within individuals, but differ systematically among individuals of different size. IPI stability was examined in photographically identified individuals recorded repeatedly over different dives, days, and years. IPI variation among dives in a single day and days in a single year was statistically significant, although small in magnitude (it would change total length estimates by <3%). As expected, IPIs varied significantly among individuals. Most individuals showed significant increases in IPIs over several years, suggesting growth. Mean total lengths calculated from published IPI regressions were 13.1 to 16.1 m, longer than photogrammetric estimates of the same whales (12.3 to 15.3 m). These discrepancies probably arise from the paucity of large (12-16 m) whales in data used in published regressions. A new regression is offered for this size range.

Echolocation Calls of the Long-Tailed Bat: A Quantitative Analysis of Types of Calls

The echolocation calls of long-tailed bats (Chalinolobus tuberculatus) were recorded in the Eglinton Valley, Fiordland, New Zealand, and digitized for analysis with signal-processing software. Univariate and multivariate analyses of measured features facilitated a quantitative classification of the calls. Cluster analysis was used to categorize calls into two groups equating to search and terminal-buzz calls described qualitatively for other species. When moving from search to terminal phases, the calls decrease in bandwidth, maximum and minimum frequency of call, and duration. Search calls begin with a steep-downward FM sweep followed by a short, less-modulated component. Buzz calls are FM sweeps. Although not found quantitatively, a broadband pre-buzz group of calls also was identified. Ambiguity analysis of calls from the three groups shows that search-phase calls are well suited to resolving the velocity of targets, and hence, identifying moving targets in stationary clutter. Pre-buzz and buzz calls are better suited to resolving range, a feature that may aid the bats in capture of evasive prey after it has been identified.

Growth and reproduction of female New Zealand sea lions

Abstract A sample of 834 female New Zealand sea lions (Phocarctos hookeri), which were aged and measured, was obtained between 1998 and 2001. In addition, the reproductive histories of 505 marked females from the Auckland Islands were recorded from 1998 to 2005. These data sets were used to investigate growth and reproductive rates. Length and weight ranged from 134 to 197 cm and 49 to 156 kg, respectively. A Gompertz growth model best described growth and predicted that females attained 90% of asymptotic length (161.7 cm) and weight (112.0) at ages 4 and 11 years, respectively. No significant differences were found in growth rates among years, nor between the 2 major breeding colonies in the Auckland Islands. Females reproduced between the ages of 3 and 26 years, with evidence of reproductive senescence starting at age 23 years. Although females up to age 28 years were observed, no females over 26 years were recorded as reproductive. Age-specific reproductive rate p(x) increased rapidly between ages 3 and 7 years, reached a plateau between ages 7 and 23 years, and then declined rapidly after age 23 years. Mean observed reproductive rate was p(x)3–28  =  0.67 (SE  =  0.01). This is the 1st robust estimate of reproductive rate for this species, is consistent with rates reported for other sea lions, and is considerably lower than assumed rates used in recent population modeling for this species. This calls into question the current method for estimating levels of sustainable bycatch. Low growth and reproductive rates are consistent with a population that is occupying a marginal foraging environment. These factors, along with a recent significant decline in pup production, suggest that current management is insufficient to ensure population stasis, let alone meet the statutory goal of recovery.

Modelling Survival of Hector's Dolphins around Banks Peninsula, New Zealand

In 1988, a marine mammal sanctuary was established around Banks Peninsula to reduce Hectors dolphin entanglement in commercial and recreational gill nets. We describe the application of a multistate mark-recapture model to data from photographic identification surveys to determine the effect of the sanctuary on dolphin survival rates. The model allows sightings to occur in three geographic strata with free movement between strata. Although we found evidence of both area and time effects on sighting probabilities we found no evidence that survival rates depended on area or time. Thus, there was no evidence that dolphin survival rates increased following establishment of the sanctuary. However, power of this test is shown to be low.

Measuring body length of male sperm whales from their clicks: The relationship between inter-pulse intervals and photogrammetrically measured lengths

Sperm whales (Physeter macrocephalus) emit short, broadband clicks which often include multiple pulses. The time interval between these pulses [inter-pulse interval (IPI)] represents the two-way time for a pulse to travel between the air sacs located at either end of the sperm whales head. The IPI therefore, is a proxy of head length which, using an allometric relationship, can be used to estimate total body length. Previous studies relating IPI to an independent measure of length have relied on very small sample sizes and manual techniques for measuring IPI. Sound recordings and digital stereo photogrammetric measurements of 21 individuals were made off Kaikoura, New Zealand, and, in addition, archived recordings of whales measured with a previous photogrammetric system were reanalyzed to obtain a total sample size of 33 individuals. IPIs were measured automatically via cepstral analysis implemented via a software plug-in for pamguard, an open-source software package for passive acoustic monitoring. IPI measurements were highly consistent within individuals (mean CV=0.63%). The new regression relationship relating IPI (I) and total length (T) was found to be T=1.258I+5.736 (r(2)=0.77, p<0.001). This new regression provides a better fit than previous studies of large (> 11 m) sperm whales.

Laser photogrammetry to determine dorsal fin size in a population of bottlenose dolphins from Doubtful Sound, New Zealand

Laser photogrammetry (also known as laser-metrics) can provide valuable morphological data but the measurement error associated with the technique has not been quantified. Here laser-metrics were used to measure the dorsal fins of an entire resident population of bottlenose dolphins (Tursiops truncatus) from Doubtful Sound, New Zealand. Dorsal fin base length, height and surface area were measured from dorsal fin photographs. Sources of measurement error were estimated by repeatedly measuring multiple photographs of dorsal fins of known individuals. Measurement error accounted for less than 6% of the total variation in dorsal fin base length and height, indicating that the technique was repeatable. Adults were the only age-class to express sexual dimorphism, with males significantly larger than females in all measurements. The relationship between dorsal fin height and base length was significantly different between male and female adults: dorsal fins of males were proportionately taller. Laser photogrammetry is an inexpensive and non-invasive measurement technique that can provide valuable size data when used in conjunction with routine dorsal fin photo-identification studies.

Designing a mark-recapture study to allow for local emigration

In using a population projection model to help manage the conservation of along-lived species, we usually need a reliable estimate of adult survival. Mark-recapture studies are often used to estimate survival, and typically require the assumption that there is no permanent emigration from the study area. We consider how such a study might be extended to allow for local emigration, that is, movement of individuals into an area peripheral to the study area. In particular, we focus on the question as to how much field effort is required in this peripheral area in order to obtain sufficiently precise estimates of both the survival probability and the probability of local emigration. We consider the use of multi-state, mark-recapture models as a means of providing these estimates and show how to assess the precision of a potential study design by calculating the expected confidence limits associated with the resulting estimates. We considera range of design scenarios for the situation that motivated this work, involving a population of Hector’s dolphins in New Zealand. For this case, it appears that there is little gain in precision once a capture probability of 0.4 is reached in the peripheral area.