Remove individual marine and freshwater mammals exhibiting aggressive behaviours that may limit population recovery

Overall effectiveness category Trade-off between benefit and harms
Number of studies: 1
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How is the evidence assessed?

Effectiveness

60%
Certainty

40%
Harms

20%

Calculating overall effectiveness category

Background information and definitions

Individual marine and freshwater mammals may exhibit aggressive behaviours that may limit the recovery of threatened populations. For example, male Hawaiian monk seals Monachus schauinslandi may attack and injure adult females resulting in high levels of female mortality and reduced reproductive success (Hiruki 1993a). Pups of both sexes may also be injured and killed (Hiruki 1993b). Removing such individuals from colonies, e.g. by translocating them to other areas, may help threatened populations to recover. However, careful consideration must be given to appropriate release sites to ensure that the threat is not transferred to other colonies.

Hiruki L.M., Stirling I., Gilmartin W.G., Johanos T.C. & Becker B.L. (1993a) Significance of wounding to female reproductive success in Hawaiian monk seals (Monachus schauinslandi) at Laysan Island. Canadian Journal of Zoology, 71, 469–474.

Hiruki L.M., Gilmartin W.G., Becker B.L. & Stirling I. (1993b) Wounding in Hawaiian monk seals (Monachus schauinslandi). Canadian Journal of Zoology, 71, 458–468.

Study locations

Key messages

One study evaluated the effects of removing individual marine mammals exhibiting aggressive behaviours that may limit population recovery. The study was in the North Pacific Ocean (USA).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (1 STUDY)

Survival (1 study): One before-and-after study in the North Pacific Ocean found that after removing aggressive male Hawaiian monk seals, the survival of adult female Hawaiian monk seals increased.
Condition (1 study): One before-and-after study in the North Pacific Ocean found that fewer female Hawaiian monk seals were injured after aggressive male Hawaiian monk seals were removed.

BEHAVIOUR (0 STUDIES)

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A before-and-after study in 1983–2005 on an island in the North Pacific Ocean, Hawaii, USA (Johanos et al. 2010) found that removing individual adult male Hawaiian monk seals Monachus schauinslandi exhibiting aggressive behaviours resulted in fewer injured and greater survival of adult female seals compared to before removal. After removal of aggressive adult males, a greater proportion of adult female seals survived each year (average 99.7% survived; total 3 seals died) than before the males were removed (average 95.9% survived; total 30 seals died). The average proportion of injured (but not killed) adult female seals each year was lower after aggressive males were removed (2%) than before (11%, numbers not reported). In 1984–1994, a total of 37 adult males exhibiting aggressive behaviours (attacking or harassing female seals) were removed from an island and either released in a different area (30 seals), were taken into captivity permanently (five seals) or died during capture/in captivity (two seals). Seals were monitored daily on the island for 3–9 months during spring and summer in each of 10 years before (1983–1994) and after (1995–2005) the removal of aggressive adult males.
Study and other actions tested
Referenced paper
Johanos T.C., Becker B.L., Baker J.D., Ragen T.J., Gilmartin W.G. & Gerrodette T. (2010) Impacts of sex ratio reduction on male aggression in the critically endangered Hawaiian monk seal Monachus schauinslandi. Endangered Species Research, 11, 123-132.

Please cite as:

Berthinussen, A., Smith, R.K. and Sutherland, W.J. (2021) Marine and Freshwater Mammal Conservation: Global Evidence for the Effects of Interventions. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Marine and Freshwater Mammal Conservation

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