Importance of biological parameters in assessing the status of Delphinus delphis

Abstract

Short-beaked common dolphins Delphinus delphis in the eastern North Atlantic (ENA) are subject to mortality due to entanglement in various types of fishing gear. However, for this region, there is no population-level information available on trends in abundance, (incidental) mortality rates or even the actual distributional range. Working under the assumption that only 1 population exists in ENA waters, the current study presents basic life history data and investigates whether biological information obtained from postmortem data is, in itself, useful for managing this population. Life history parameters were estimated by analysing postmortem data obtained over a 16 yr period by UK, Irish, French, Galician (northwest Spain) and Portuguese stranding and bycatch observer programmes. An annual pregnancy rate of 26%, a calving interval of 3.79 yr, an average age attained at sexual maturity of 8.22 yr and an average length at sexual maturity of 188 cm were determined. With respect to the findings based solely on mortality data, significance testing failed to detect differences that could be construed as evidence of the population exhibiting what might be density-dependent compensatory responses. The low annual pregnancy rate reported throughout the sampling period may suggest either that the level of anthropogenic mortality did not cause a substantial population level decline, or a prey base declining at approximately the same rate as the dolphin population. However, this approach alone does not facilitate an assessment of the current state of the D. delphis population in the ENA. Population abundance estimates, trends in abundance and knowledge of factors that affect the dynamics of the population, such as annual mortality rates in fisheries, temporal variations in prey abundance and effects of contaminants on reproductive activity, are required not only to set management objectives, but also to give context to cross-sectional life history information.

Assessing the effect of persistent organic pollutants on reproductive activity in common dolphins and harbour porpoises

“…comm. ), whereas the APR for D. delphis is similar to that proposed for the ENA population (26%; Murphy et al, 2009b). These data suggest that harbour porpoises are pregnant more of the time than common dolphins, and would therefore possibly ovulate less; common dolphins are more likely to undergo repeated unsuccessful ovulations during their extended calving interval.…”

“…Although these sample sizes are small, results do not suggest a CA half-life of less than one year; taking into account the estimated lactation period for this population is 10.4 months , a gestational period of ca 12 months (Murphy et al, 2009b) and reproductive seasonality i.e. not ovulating outside the mating season.…”

“…They were unable to differentiate CAs from pregnancy with those from ovulation, and it was assumed that smaller CAs, containing <15% elastin, were derived from infertile ovulations. With a reproductive period of ca 19 years reported for the ENA D. delphis population (Murphy et al, 2009b), results from Takahashi et al (2006) study suggest that CAs, of pregnancy at least, do persist throughout the lifetime of a female common dolphin.…”

“…In UK waters, female P. phocoena attain sexual maturity at ca 4.5 years and the calving interval is ca 2 years Murphy, MS 2008) compared to 8.2 years and 3.8 years, respectively, in D. delphis (Murphy et al, 2009b). A low pregnancy rate of 26% has been reported for the ENA D. delphis population, and although it has been suggested that the pregnancy rate may well in fact be the natural rate for this species in a temperate region, it cannot be ruled out that environmental and other anthropogenic activities, such as chemical and physical pollutants, may be contributing factors to the low reproductive output (Murphy et al, 2009b). Further, P. phocoena in UK waters exhibit among the lowest pregnancy rates reported for this species; ranging from 34% in Scottish waters (J.…”

Deposition of growth layer groups in dentine tissue of captive common dolphins <i>Delphinus delphis</i>

“…30-years ago by Gurevich et al (1980) through the use of tetracycline, an antibiotic that was employed as a fluorescent vital marker in teeth of captive dolphins. Estimation of age in common dolphins is primarily based on counting GLGs in the dentine (Murphy and Rogan 2006, Danil and Chivers 2007, Westgate and Read 2007, Murphy et al 2009) rather than the cementum tissue, as the latter can be too thin to allow resolution of GLGs . did use counts of GLGs in the cementum for verification of GLG based estimates in dentine tissue, though actual results of this comparison in accumulation of incremental layers was not presented.…”