Aspects of reproduction and seasonality in the harbour porpoise from dutch waters

9 1995 Elsevier Science B.V. All fights reserved Whales, seals, fish and man A.S. Blix, L. WallCe and 13. Ulltang, editors

459

Aspects of reproduction and seasonality in the harbour porpoise from Dutch waters M.J. Addink 1, T.B. SCrensen 2 and M. Garcfa Hartmannl,3, 4 1National Museum of Natural History, Leiden, The Netherlands; 2Cell Biological-Anatomical Laboratory, Zoological Institute, University of Copenhagen, Copenhagen, Denmark; 3Seal Research and Rehabilitation Centre, Pieterburen, The Netherlands; and 4 Duisburg Zoo, Duisburg, Germany Abstract. Background: since about 1960 the harbour porpoise Phocoena phocoena (L.) in Dutch waters has declined in number. In 1990 a research programme was started to establish the possible causes of this decline as well as to obtain basic information on the species' life history. Methods: a complete postmortem was carried out on most stranded animals and some known by-catches. The reproductive organs were studied. Historical data were investigated for useful reproductive information. Results: near-term foetuses were observed in March and April; neonates from May to the end of August. Spermatogenetic activity was observed in one male in March. In a sample of seven females aged at 3 5 years, five were sexually mature. Within a sample of 15 mature females, four had both ovaries functionally developed. Conclusiens: the reproductive season and birth period in Dutch porpoises appear to be extended in comparison with some other populations. A previous study found an age of sexual maturity (ASM) of 6 years for Dutch female porpoises. In the present, limited study the ASM appears lower. Although Phocoena is known for its ovarian asymmetry, 4 out of 15 (26.6%) of the females showed functional development of both ovaries. Key words: Phocena phocena, The Netherlands, female age at sexual maturity (ASM), ovarian asymmetry

Introduction From about 1960, the number of stranded harbour porpoises Phocoena phocoena (L.) severely declined on the coasts of the southern North Sea [1-4]. Sighting records, even though many of these are anecdotal, show a similar trend [4,5]. In 1990 an intensive research programme was started in the Netherlands, with the aim of gathering information on the possible causes of this decline and to obtain basic information on the harbour porpoise. The programme collects data on life history, pathology, pollutant burden and stomach contents.

Materials and Methods All porpoises Collected by the stranding network of the National Museum of Natural History in Leiden are studied during a thorough postmortem. The dissection protocol

Address for correspondence: M.J. Addink, National Museum of Natural History, P.O. Box 9517, 2300 RA Leiden, The Netherlands.

460 is based on an expanded version of the European Cetacean Society dissection protocol for small cetaceans [6]. During the dissection the uterus and ovaries or the testes and epididymides are examined and fixed in 10% neutral buffered formalin. When a foetus is present, its length and weight are recorded. The ovaries are studied under a binocular microscope, and the numbers of corpora lutea (CL) and corpora albicantia (CA) are counted. A porpoise is considered sexually mature if an ovary contains at least one CL or CA. When a CL is present but no foetus is found, the uterus is examined for signs of recent pregnancy. Histology is used to confirm lactation. So far, the testes and epididymides of two animals have been studied histologically. Each testis sample was dehydrated in acidified 2,2-dimethoxypropane, cleared in toluene, embedded in Paraplast and sectioned at 8 r Sections from each sample were stained with Ehrlich's haematoxylin and eosin and mounted in entellan prior to examination. The remaining samples will be processed later. Teeth were processed using standard techniques [7] and age estimates were obtained from counts of growth layer groups. Since about 1920, Dutch biologists have collected porpoises for anatomical research. For the present study, published records [8-11] as well as archives and other historical records were investigated for useful reproductive information.

Results

As the research is still in progress, only aspects of seasonality in birth period and male reproduction are presented here, as well as certain data concerning female age at sexual maturity (ASM) and ovaries. Data on lactation, male ASM, etc. will be presented elsewhere. These are preliminary results based on various subsamples of the database, which were ready for interpretation. The average length at birth of 27 porpoises which, according to biological and/or pathological records were definitely neonate, is 74.3 cm (SD 7.8 cm; range 6397 cm). Apart from these, there is a much larger dataset giving reliably recorded lengths but no information on whether the animals were newborn. Because no statistical tests have yet been carried out on the data, for this article only the above 27 porpoises definitely neonate, and those (n = 47) from the other dataset measuring <74.3 cm (the average length at birth), were used to determine the months of birth. Dutch porpoises show a prolonged birth period (see Fig. 1) which extends into August. Other evidence suggesting an extended mating and, by inference, birth period is that in two males, testis activity was found outside the reproductive period (JuneSeptember) given for Denmark [12]. Gross pathological examination showed that a male found stranded on 26 March 1992 had the epididymides full of sperm (see Fig. 2), and a male found on 6 November 1990 had some traces of sperm. Histological examination revealed spermatogenetic activity in the male from March, but no sig-

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Fig. 1. Near-term foetuses and neonates of Phocoena phocoena from Dutch waters, by month of birth. Although there is a clear peak in July, the birth season extends into August.

nificant activity was found in the male from N o v e m b e r (this specimen requires a more thorough study). Seven female porpoises studied for reproduction were aged as b e t w e e n 3 and 5 years old. Table 1 shows that five of these were already sexually mature. The ani-

Fig. 2. Sperm taken from the epididymides of the male stranded on 26 March 1992, photographed during the postmortem (photograph J.C. den Hartog).

462 Table 1. Female Dutch harbour porpoises aged between 3 and 5 years old

Date 89.03.15 89.09.01 90.12.05 91.01.02 92.01.09 92.01.18 92.11.22

Place

Maturity

Bloemendaal Mature Z a n d v o o r t Mature Oosterschelde Immature Texel Mature Texel Immature Camperduin Mature Castricum Mature

Length

Weight (kg)

Age (year)

Pregnant

(cm) 145 154 132 169 149 147.5 150

59 39 29 51 33.5 48 41

5-5.75 3-4 4.25 5 5 5.25 5

Yes No No No No

Five out of seven porpoises are already sexually mature (defined as at least one CL or CA present). mal stranded on 15 March 1989 was pregnant with a 53 cm foetus. It had only one CL of pregnancy and no other scars, so must have become pregnant for the first time when it was between 4 and 5 years old. Although P h o c o e n a is known for its ovarian asymmetry, within a series of 15 sexually mature females with at least one CL or CA, four animals showed functional development of both ovaries (26.6%). One of these, a pregnant animal, had the foetus in the right uterus horn and the CL of pregnancy in the right ovary.

Discussion Although there is a pronounced peak in July, the birth period in Dutch waters appears to be extended (see Fig. 1). SCrensen and Kinze [12] as well as Read [13] report a narrow birth peak, from the middle of June to early July for Denmark and from May to June for Canada. In central Californian waters, however, there appears to be a more extended birth period again, from June into August/early September [ 14]. There are sightings of calves as early as April in British waters (P.G.H. Evans, personal communication) and there is one calf in April among a limited set of sightings in Dutch coastal waters [15]. The causes of these differences between porpoises from different geographic areas are not clear; for a discussion of the topic, see SCrensen and Kinze [ 12]. The studies from Denmark and Canada [12,13] also find a strongly synchronized female reproductive cycle, and this appears to be true for males as well. Because most Dutch porpoises are collected in late autumn/winter, there are almost no females available for studying maturing follicles. So far, based on gross pathological examination, only ten males assumed to be mature have been collected during this study. The two males with aseasonal (as defined by SCrensen and Kinze [12]) sperm activity would be in agreement with an extended female mating period. A previous study found an age of sexual maturity of 6 years for Dutch female porpoises [ 11 ]. In our material, the ASM is lower for five of the seven animals studied (see Table 1). It is suspected that some of the ages published by Van Utrecht [ 11 ] may be too high. Some of the teeth studied by Van Utrecht were cut in a way that

463 could lead to an erroneous age determination (C. Lockyer, personal communication). The animals in question will be aged again. If Van Utrecht's age determinations prove correct, our findings could be evidence of a population under pressure, in which the ASM would have decreased by 1 year or more. A similar reaction was observed in the harbour porpoise population in the Bay of Fundy, Canada [ 16]. Phocoena is known for its ovarian asymmetry, with only the left ovary developing during puberty and all activity confined to this ovary [12]. It is thus very interesting that 4 out of 15 females from Dutch waters have both ovaries functionally developed. Because histological examinations are not completed, we will refrain from speculating about the possible causes of this phenomenon.

Acknowledgements We thank P.J.H. van Bree for giving access to historical reports and archives, T. Olesen for processing the two samples for histology, H. Kremer for processing the teeth, J.C. den Hartog for taking photographs during a postmortem, and C. Smeenk and P. de Wilde for critically reading this manuscript.

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