Dead, beached fish a mystery

The cause of death of a large
number of fish along the Seven Mile Beach waterline on Wednesday has so far
eluded environmental officials.

“We’re at a loss to figure out what
would cause the deaths,” said John Bothwell, senior research officer with the
Department of Environment, which is investigating the matter.

DOE officials said the fish appear
to be file fish about 2 to 3 centimetres long. Although the officials believe
the fish are white-spotted file fish, due to the juvenile stage of the fish, it
was not possible to conduct a definitive identification of the species.

“Marine fish die-offs like this are
unusual and without an obvious causative agent, for example a recent chemical
spill or flooding event, or obvious signs of distress, for example burst
internal organs from dynamite fishing or heavy parasite loads,” said Mr.
Bothwell.

He estimated that only about three
such incidents have occurred in Cayman in the last 10 to 12 years.

“This is different from the
unfortunately common event of introduced invasive tilapia dying off in ponds
over summer when the oxygen level naturally drops for one reason or another,”
said Mr. Bothwell.

In spite of the large number of
dead fish, Mr. Bothwell stated that the department did not suspect that there
was a systemic environmental problem at this time.

Sample fish were taken and have
been preserved for possible future examination. The department has also posted
a request for information on possible causes and assistance with identification
of the species on the Gulf and Caribbean Fisheries Institute.

According to Mr. Bothwell, file
fish are a mid-level omnivore on the reefs. They mostly eat small crustaceans
and algae but probably also scavenge anything else they can find. However,
because they grow relatively large, are relatively tough-hided and tend to stay
close to the reef, the adults are not commonly considered a likely prey species
for many upper-level carnivores.

“This makes them fairly important
as a balance on the reef,” said Mr. Bothwell.

“Although not numerous, there’ll
always be a few file fish around to help keep the reef healthy. Juvenile file
fish are seldom observed. However, if the numbers in this fish kill are any
indication, they are probably a not inconsequential forage fish for small
predatory fish,” he said.

According to Mr. Bothwell, it is
quite possible that the file fish spend their juvenile phase at sea in
association with sargassum seaweed and other floating objects. In these
circumstances they would form prey for young pelagic predators, such as baby
dolphin and tuna. Once settled out onto the reef, they would be preyed upon by
other small predators, including grunts, squirrel fish and snappers, which in
turn are preyed upon by larger predators such as barracudas, sharks and large
groupers. This would continue until the file fish grew large enough to cease
being a viable prey.

Mr. Bothwell said it is possible
that the dead juveniles belonged to a year-class recruiting to Cayman that died
during the transition from pelagic juveniles to cryptic reef dwellers.

However, he said he does not
believe this would have an appreciable impact on the number of adult file fish
observed in Cayman, as he expects many of them would have fallen victim to
predators as they matured.

“We don’t know how often file fish,
or any other local species with a planktonic larval stage, recruit to Cayman.
It is possible that some years there is good recruitment and many years recruitment
is low. However, if this sporadic recruitment is the case, and we don’t know
that it is, it is possible that in a few years we could have a noticeable
reduction in the number of file fish on our reefs,” said Mr. Bothwell.

Although such an event could have a
negative impact on the reef, Mr. Bothwell said such events point to the need to
keep entire ecosystems generally healthy, as a healthy ecosystem is less likely
to be affected by such one-off events. It is probable that such events occur
often enough over the life of a reef that a healthy population of file fish can
handle the loss of even a good recruitment year.

“It’s one of the frustrating things
for people that wildlife managers often can’t affect these sorts of events and
so have to focus on other things we can control, like fishing pressure through
marine parks or anchor damage through moorings, to increase ecosystem
resiliency to whatever does happen,” said Mr. Bothwell.

Checks conducted by the department
along other coastlines around the Cayman Islands have not found any other
incidences of fish die-offs, but the public is urged to report sightings of
large numbers of dead fish to the department at 949-8469 or doe.gov.ky.

LOCALDeadfishmysterySTORY

Dead juvenile fish litter the waterline.
Photo: Submitted
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