The Problem with Fleas
By Steve Kaimmer
Sand fleas are parasitic crustaceans common throughout the North Pacific. They are amphipods, rather than copepods. They attach themselves to other water creatures by digging into the flesh or scales and eat at the animals they are attached to. Small fish, swordfish, sunfish, flying fish, starfish, and even whales are attacked by the different kinds of parasitic crustaceans.
Flea size can vary from as small as a short grain of rice to 3 or 4 times larger. Their distribution is spotty, and they seem to be more common on sandy bottom. ‘Hot’ spots in their distribution may change from season to season or year to year. These may be very localized, such that a part of a set can be severely impacted by fleas, while another part shows no damage. Fleas appear to be more active at night, and the effect on captive fish is also more evident on longer sets. These seem to be additive, so that a long night set might have worse predation than a short night set, etc.
Flea predation starts with the presence of fleas on the surface of the body. They appear to first remove (eat?) the slime, then the top layer of the skin. The fleas seem to enter the body first through holes eaten through the skin membrane either near the eye, anus, or dorsal fins. First evidence of fleas (other than their physical presence on the body) can be a non-glossy whitened appearance in these areas where the slime and scales have been eaten. This is still a non-fatal condition.
Once the fleas have penetrated the body, we consider the fish to be dead. Penetration near the anus or eye is often evident by the presence of a hole about .25 centimeter in diameter. Fleas entering in these locations probably go first for the ‘sweet meats’ at the base of the gill attachments or the gonads. Halibut are quickly consumed once fleas invade. The membranes of the dorsal fins are often eaten, and entrance holes can be seen near their base. The dorsal and/or anal fin membranes may be eaten away, leaving fin rays exposed. Skin on the body is separated from tissue where sand fleas have eaten. At this stage, moving bumps can often be seen under the skin near the dorsal fin where fleas are moving about. After eating sweetmeats and gonads, the blood from the bloodline below the spine and from the gill arches disappears. The fleas then consume the muscle tissue. At this point, a fish may still be attached to the hook and a bag of skin and bones may be retrieved on the longline. From observations of halibut caught by pots and attacked by fleas, the skin seems to be the last thing eaten.
