Abstract

Reproduction control in captive pinnipedia is an important subject for most facilities. So far reproduction control in harbour seals (Phoca vitulina) has been achieved by antiandrogens, gestagen preparations and castration. Although they result in efficiently lowering testosterone levels, most of the preparations do have severe side effects on the animals or and castration is impossible to reversible. Due to results obtained in healthy adult harbour seal males, testosterone levels rise in march/April, a peak in May/June and a decrease in September. Here we present the result of the use of a depot gonadotropin-releasing hormone (GnRH) preparation (Suprefact®-3-Months-Depot, Burserelin) in a harbour seal in regard to its efficacy to achieve temporary testosterone suppression when given before the annual testosterone peak.

The harbour seal group at the Seal station in Friedrichskoog consists of 3 mature females, one mature and one immature male. The mature male received a subcutaneous Suprefact®-3-Months-Depot (9,9 mg, Buserelin) implant one April 23th, 2000 and 14 days later (May 4th, 2000). Testosterone levels were measured from serum samples before injection, 12 days after the first injection, and then monthly by a routine radioimmune assay.

The testosterone level before the first injection was below the detection limit (<0,02 ng/ml). On day 12 (after the first injection), there was an increase to 0,08 ng/ml. Monthly measurements thereafter were negative. In September (4 months after the second injection), a low level of testosterone (0,05 ng/ml) was detected. No behavioural changes such as social ranking were observed. Sexual activity was reduced during the experimental period but did not completely disappear.

Based on our findings, a single injection of Suprefact®-3-Months-Depot does not achieve a full suppression of testosterone, but a second injection, when given 14 days later, resulted in a complete suppression of testosterone for almost 4 months. Thereafter, the animal's testosterone levels returned to normal. Therefore, Suprefact®-3-Months-Depot is an effective drug that can be used to control reproduction in harbour seals without side effects and behaviour changes.