Monitoring growth of harbour porpoise (Phocoena phocoena) in human care

Transcription

1 ICES CM 2001/J:29 Monitoring growth of harbour porpoise (Phocoena phocoena) in human care 1 Christina Lockyer, 2 Genevieve Desportes, 2 Kirstin Anderson, 2,4 Sabrina Labberté and 3 Ursula Siebert 1 Danish Institute for Fisheries Research, DK 2920 Charlottenlund, Denmark. Tel: ; Fax: ; chl@dfu.min.dk 2 Fjord and Belt Centre, DK 5300 Kerteminde, Denmark. 3 Forschungs- und Technologiezentrum Westkueste, Christian Albrechts University of Kiel, Buesum, Germany. 4 Currently of Via Donatello 11, Milano (MI), Italy. Abstract A male and female harbour porpoise, taken into human care at the Fjord and Belt Centre in April 1997, are housed in an outdoor pool area off Kerteminde fjord, Denmark. The estimated age was 2-3yr for both animals so that current ages are 6-7yr (April 2001). Growth has been monitored since capture by means of body length and weight, girth, blubber thickness, and dietary intake. Each food batch has been analysed biochemically. Their weights have fluctuated seasonally during the period, with increasing weight from October, peak in January, and rapid loss in spring around April. Girth and blubber thickness mirror these weight fluctuations. Food intake has also fluctuated seasonally, but increases have preceded weight gains. Daily food consumption ranged from kg (ca 7 9.5% body weight). During the 48 months since capture, lengths have increased steadily from cm (male), and from cm (female), and initial weights from 37.5 max.47 kg (male) and 40.5 max.60 kg (female). Body lengths and weights compare favourably with expected body sizes determined from by-caught porpoises in the region. We anticipate that seasonal changes in body fat are correlated both with water temperature and summer reproductive activity, suggesting that blubber fat may serve as insulation and energy reserves.

2 Introduction Two harbour porpoises, Eigil (male) and Freja (female) were taken into human care on 7 th April 1997, after rescue from a pound net set in the Inner Danish Waters. These types of net are commonly set from the shore in shallow coastal waters, and porpoises that become entrapped are normally alive and in good condition when discovered within a few days. The two rescued animals are presently housed at the Fjord and Belt Centre, Kerteminde, in a seminatural outdoor pool, which is a penned-off area of Kerteminde fjord. This pool is approximately 15m by 36m and 3.5m deep with both rock and sandy bottoms, and the open system allows entry of local fauna and flora, including smaller sized fish. The system is also exposed to the climate, the tidal flow and normal sea (fjord) temperatures. The main purpose of holding the animals was for research into life history, feeding and behaviour that are relevant to the problem of incidental capture in bottom-set gillnets in Danish waters. The aim of this particular project has been to assess growth and feeding efficiency in harbour porpoise, and learn more about this species physiology. Methods Growth Parameters The project commenced shortly after the arrival of the two animals in April 1997, and growth has been monitored by means of the parameters of total body length, body weight and blubber thickness (Lockyer, 1995a) as well as dietary intake by weight and dietary composition (Lockyer et al, 1999). Initially these parameters were taken almost daily, until the animals were feeding satisfactorily, then weekly and then presently every two or four weeks. The early frequent measurements were because of the need to constantly monitor the health of the animals. Body weights were always attempted at the same time of day, and in the first days were often taken both morning and evening. During these procedures the animals were removed from the water by means of a special stretcher, and brought to an examination table for measuring and weighing which is performed on a large platform balance with an accuracy to.01 kg. Length (taken in a straight line from jaw tip to tail fluke notch, parallel to the body) and six girths (see Lockyer 1995a) are measured manually by means of a soft tape measure. The blubber thicknesses were measured dorsally, laterally and ventrally in three positions along the body by means of a portable ultra-sonic fat scanner (Lean Meter as used in pig husbandry). Fig. 1 demonstrates all these measuring sites.

3 In recent months training has permitted blubber thickness to be measured poolside while the animals remain in the water. This is less stressful for the animals and appears to provide values within the usual expected range. We have also attempted to measure total length poolside with assistance of two helpers to determine the snout and tail fluke notch positions on the pontoon while the animal remains in the water. The length, thus marked out, is then measured with a tape. The animals are also training to beach onto a weighing platform, so that the trauma of being lifted from the water is avoided. Food consumption All fish were quality-controlled, of within a certain size range (10-28 cm length), kept frozen and only thawed for use as required. The fish were weighed and recorded daily as food intake and species. A sample of fish from each new food batch was retained for biochemical analysis. The diet has been predominantly mackerel (Scomber scombrus) and herring (Clupea harengus) with occasional cod (Gadus morhua), and live trout (Salmo spp.) used in some experiments. Results and Discussion Growth Parameters The first year was regarded as a settling-in period, especially the first few months. The parameter measurements were found to be variable and sometimes difficult to measure because even small movements of the animals (breathing) could alter girths and body flexing could alter length by as much as 3 cm. Therefore, there was a learning period to establish criteria for accurate measurement recording. The blubber thickness was also difficult to measure on the ventral surface because the animals were distressed when turned slightly to access the underside. The ideal is to train the animals to present themselves in certain postures while still in the water, and this has been progressing well in recent months, thus hopefully eventually eliminating the need to remove the animals from the water for this investigation. During the 48 months since capture, lengths have increased steadily from cm (Eigil), and from cm (Freja) (Figs 2 and 3). The estimated ages at first capture were about 2-3 yr for each animal. The current lengths of the animals are as estimated for a wild animal from this region and about age 6-7 yr (Lockyer and Kinze, 1999). The initial period in captivity resulted in major weight losses, especially in the dorsal thoracic and trunk regions because of refusal to feed. Such losses were sudden and dramatic in just the first few days, with about 5 kg being lost by Freja and 4 kg by Eigil who continued

4 to lose a further 2.5 kg until day 60 (Figs 4 and 5). Initial body weights were 40.5 kg for Freja and 37.5 kg for Eigil. Once feeding was established, weight stabilised within the first 60 days, but noticeable weight increase and growth in length did not take place until the animals were transferred outside and finally freed into the entire pool space. A stable period in body weight continued up to about 180 days (about 6 months) after capture, and then increased steadily over the next few months during winter, reaching a peak of 51.6 kg for Freja and kg for Eigil in late January/early February Body weight then diminished with weights of 47.2 kg for Freja and kg for Eigil in July Since July 1998, body weights have continued to fluctuate markedly with season, but have generally shown an increasing trend over time (Figs 4 and 5). The weights have fluctuated regularly, with weight increasing from October, peaking in January, remaining high throughout winter and then been lost rapidly in spring around April with a low weight during summer. This pattern is demonstrated clearly for the past four years (Figs 4 and 5), and one may assume that this is a regular intrinsic natural seasonal fluctuation. There is rather less evidence for an overall net increase in body weight in recent times, implying that growth in body weight has reached a plateau. Overall, body weights have increased from initial 37.5 kg to max. 47kg (Eigil) and 40.5 kg to max. 60kg (Freja), adjusting for seasonal fluctuations. The body weights compare favourably with expected weights for length and age as determined from by-caught porpoises in this region (Lockyer and Kinze, 1999, Lockyer, 1995b; c). The initial weight loss on captivity in 1997 is not unexpected because of the lack of feeding at first. However, the losses were not expected to be so sudden. The losses were characterised by rapid concavity of the tissue behind the head along the anterior dorsal region. This suggests that the energy reserves of the animals may only be short term. The large seasonal weight increases in winter months with the cold water temperatures also suggests that energy reserves and blubber fat in terms of insulation may be important. The measurements of girth and blubber thickness (Figs 6 and 7; Figs 8 and 9 respectively, which show mid-girth, G3, and mid-dorsal blubber thickness, D3, for Freja and Eigil) generally correlate well with observations on weight, so that the interpretation of seasonal fat deposition appears most likely. As the porpoises have grown and matured, the blubber thickness appears overall not to have increased and actually appears to be less than earlier levels observed. This is especially evident in Freja (Fig. 9), even though there are dramatic seasonal increases. Studies on body composition of wild porpoises (Lockyer, 1995a; c) have shown that immature animals have the thickest and relatively largest amount of

5 blubber reserves, so that these declines overall in blubber thickness with age are in line with expectations and are not abnormal. Food consumption The average daily food consumption is shown in Figs 10 and 11. The daily food consumption generally varies between kg for Freja and 3 4 kg for Eigil, although smaller and larger amounts may be taken. This currently represents about % body weight for Freja and % body weight for Eigil. Energetically the calorie intake is large because both herring and mackerel are fatty fish. Our own analysis of energy density of the fish fed to the porpoises showed large variations by season and also according to the fish size in herring. However, energy density of wet weight of fish varied between kjoules. g -1 for cod, kjoules. g -1 for mackerel, 6.09 kjoules. g -1 for trout and kjoules. g -1 for herring. The content of a predominantly whole herring and mackerel diet may thus vary from kjoules. g -1 so that daily food consumption could represent extremes of 17,255 35,505 kjoules (4,130 8,490 kcal.) for Freja and 14,790 31,560 kjoules (3,540 7,550 kcal.) for Eigil. The seasonal nature of the food consumption in terms of weight is very evident in Figs 10 and 11. The increase in food consumption precedes the weight increases in winter, but the cycles are just as regular as the weight variations. Conclusion In conclusion, we are now certain that there is an important seasonal weight increase largely because of fat deposition in the blubber, and this has not been documented before for porpoises in the wild, and never for individual animals over a long period of time. There is a definite link between food intake and weight gain although there is a delay between food increase and weight gain. We anticipate that from field studies of blubber thickness in porpoises (Lockyer, 1995a), both Eigil and Freja will not continue to increase overall blubber thickness, and it will diminish as it has already started in Freja, although we anticipate that seasonal variations will continue.

6 References Lockyer, C. 1995a. Aspects of the morphology, body fat condition and biology of the harbour porpoise, Phocoena phocoena, in British waters. Rep.int.Whal.Commn (Special Issue 16): Lockyer, C. 1995b. Investigation of aspects of the life history of the harbour porpoise, Phocoena phocoena, in British waters. Rep.int.Whal.Commn (Special Issue 16): Lockyer, C. 1995c. Aspects of the biology of harbour porpoise, Phocoena phocoena, from British waters, pp In, Whales, seals, fish and men, eds A.S.Blix, L.Walløe and Ø.Ulltang, Elsevier, 720pp. Lockyer, C. and Kinze, C Status and life history of harbour porpoise, Phocoena phocoena, in Danish waters. Working paper submitted to ICES Working Group on Marine Mammal Habitats, March 1999, Copenhagen, ICES/WGMMHA/99/WP19, 39 pp. Lockyer, C., Desportes, G., Anderson, K., Labberté, S., Siebert, U How well we grow monitoring growth of harbour porpoise in captivity. European Research on Cetaceans 13:

7 Total Body Length (cm) Fig. 1. Diagram showing the sites of measurement on the porpoise body: total body length form snout tip to tail fluke notch; girths (G 1 G 6 ); blubber thickness (D 2 - D 4, L 2 - L 4, V 2 -V 4 ).

8 EIGIL Fig. 2. Growth in body length over time for Eigil FREJA Fig. 3. Growth in body length over time for Freja

9 EIGIL Fig.4. Growth and seasonal variations in body weight for Eigil FREJA Fig.5. Growth and seasonal variations in body weight for Freja

10 EIGIL Fig. 6. Seasonal variations in body mid-girth G3, for Eigil FREJA Fig. 7. Seasonal variations in body mid-girth G3, for Freja

11 EIGIL Fig. 8. Seasonal variations in mid-dorsal blubber thickness D3, for Eigil FREJA Fig. 9. Seasonal variations in mid-dorsal blubber thickness D3, for Freja

12 EIGIL Fig. 10. Average daily food consumption by month, for Eigil FREJA Fig. 11. Average daily food consumption by month, for Freja