Humpbacks vs. Herring (fwd)

From: Mike Williamson (pita@www1.wheelock.edu)
Date: Mon Apr 17 2000 - 17:07:09 EDT


---------- Forwarded message ----------
Date: Sun, 16 Apr 2000 09:46:05 -0400 (EDT)
From: "Phillip J. Clapham" <pclapham@whsun1.wh.whoi.edu>
To: David Jones <davjonz@zoolink.com>, pita@whale.wheelock.edu,
    kburnett@whale.wheelock.edu
Subject: Re: Humpbacks vs. Herring

Hi:
I'd say that any discussion of how herring have evolved to react to
humpbacks is just speculation, since we really don't know; asking a herring
biologist about the general behavior of herring would be a good idea. But
first...
Bubble feeding behavior takes a number of forms. Bubble nets consist of a
series of bubble columns blown to form a circle or spiral; the whale times
the net so that it is lunging through the center as the net closes. Nets
are used extensively in the North Pacific population. A bubble cloud is
different: it's a single (sometimes multiple) burst of bubble, released
from the mouth, which forms a huge cloud sometimes 20 m or more across.
This seems to be a North Atlantic specialization; it's unknown in the N
Pacific. Most N Atlantic humpbacks use clouds, around 10% use nets. Both
probably work in more or less the same way, though the precise architecture
of the net is very different from the cruder approach of the cloud. Both
are used here in the Gulf of Maine on schooling fish like herring and sand
lance, particularly the latter.
>From having looked at thousands of bubble feeding events, I'd guess that
they work as follows. The general response of schooling fish to threat is
to bunch u more tightly; this makes it much harder for most predators to do
what they want to, which is to single out one fish, so it's a great
response to most predators. But it's a lousy response to whales, who are
going for the entire school (or as much of it as they can get). I'd guess
that bubble feeding performs two functions: first, it elicits the bunching
behavior that tightens the school, and second it creates a physical barrier
that the fish can't easily pass, which also works to keep them together.
The result is that the whale gets more fish at the end of the event when it
lunges with its mouth open into the middle of the bubble structure.
Phil Clapham



This archive was generated by hypermail 2b30 : Sat Aug 04 2001 - 10:40:12 EDT