Subject: Info: Ocra bent dorsal fins

Michael Williamson (whe_william)
Mon, 15 Feb 1995 22:01:23

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Date: Wed, 15 Feb 1995 18:15:34 -0500 (EST)
From: Michael Williamson <WHE_WILLIAM@FLO.ORG>
Subject: Info: Ocra bent dorsal fins
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Subj:	Killer whale dorsal fins
Date:         Wed, 15 Feb 1995 11:18:06 -0800
Reply-To:     Marine Mammals Research and Conservation Discussion
Sender:       Marine Mammals Research and Conservation Discussion
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From:         Robin Baird <rbaird@sol.BITNET>
Subject:      Killer whale dorsal fins
To:           Multiple recipients of list MARMAM <MARMAM@UVVM.BITNET>
In the Pacific northwest region (B.C., WA, AK) about five different
individual adult male killer whales with bent over dorsal fins have been
recorded. In these cases, the bent over fin appears to be a permanent
problem, that is, these individuals have had bent fins for periods ranging
from 2 to over 10 years. In addition, Rich Osborne (from the Whale
Museum, Friday Harbor, WA) has mentioned to me an incident in the 1970s
where a killer whale was seen with numerous fresh tooth rakes on his fin,
which was bent over. Over the next while (I can't remember the exact time
frame) the fin gradually straightened up, and this individual has a
normal fin today. This latter incident suggests that trauma can certainly
be a cause of fin bending. When the dorsal fin of a male undergoes a
period of rapid growth, usually in their early teens, their fin also
becomes much more wobbly. I suspect that during sickness, or periods when
killer whales are unable to feed, that the dorsal fin may also lose some
of its rigidity. In the wild, the forces of the water on the fin are
relatively equal on each side, so when the fin regains its rigidity it
likely does straighten up. In captivity, due to the constraints on pool
size and shape and decreased swimming speeds and activity levels, as
male dorsal fin growth is accelerated during puberty, or if a killer
whale becomes sick or stops eating for a while, the fin may lose some of
its rigidity, but when it regains its rigidity the fin has bent a bit to
one side. Over the space of many years, as these events continue to
happen, the fin becomes progressively more bent over. In captivity, this
occurs more in males than in females, presumably because male fins are so
much larger. The disproportionately large fins of adult males presumably
came about through sexual selection, thus the fin may be more subject to
damage, injury or bending then the fins of females (where fin size and
shape are likely more adaptive, in terms of natural selection).