I’ve come to the conclusion that a lot of anglers are paranoid. I imagine them waking in feverish sweats as black feathery and brown furry monsters zoom through their nightmares. Cormorants and otters surface in their watery imaginations with giant carp and ten-pound barbel hanging from their toothy maws. They believe hordes of illegal otter releasers are touring the country with trucks full of whiskered fish destroyers, which just happen to home in on their favourite species. (They don’t seem to think cormorants are being released, however.) Barbel enthusiasts think otters only eat barbel; carpers lament lutra’s taste for overfed farmed commons, trout anglers . . . well, you get the picture.
In my last post on the subject of otters, I pointed out the facts regarding otter introductions. I won’t repeat them here, other than to note the only otter introductions (last ones in the 90s) were limited. A few illegal drops may have occurred but this was very unlikely to be widespread; you can’t buy otters down the pet shop. Yet you can’t convince the otterphobes. Like all good conspiracists, their fantasies overwhelm any sense of the probable or awareness of the facts. If they are told, for example, that spraint surveys show that otters don’t eat barbel, they say that the species has already been wiped out. If you point out that barbel catches declined before otters came on the scene they claim catches only fell the moment someone mentioned otters.
Science is anathema to these anglers but for those who prefer reason and fact it’s worth seeing what research is out there (I exclude the unscientific anecdotal stuff written by anglers). As with cormorants there is not very much done in England. From the handful of papers I’ve found, there are some interesting observations. Some work from Oxford on the upper Thames found that otter diet comprised 19% fish of sporting value, that is fish anglers would be interested in catching. Surprisingly perch and pike are preferred over cyprinids, to which family carp and barbel belong. 80% of prey are between 4 and 13 cm; only 3% are over 20 cm. Crayfish comprise 14% of an otter’s diet, something which ought to please most fishermen. In summer bullhead were the major prey species (70%). These observations are supported by the other studies on otter diet.
Apart from special circumstances, as of an otter let loose on a carp lake full of large, sedate fish, the evidence suggests that otters eat mainly small fish, and falls in stocks of barbel cannot be blamed on them. A debate of this nature is currently simmering on another forum, barbel.co.uk, which unlike most forums has a cohort of sensible individuals to challenge the wilder speculations. Otters are protected and there is absolutely nothing anglers can do to change that. The otter is an indigenous creature which is returning to our rivers because we’ve stopped killing it. Damage to fish stocks is anecdotal, there is little if any hard evidence.
Finally let’s have a look at the EA survey data for the whole of England since 1980. This is not a wholly reliable measure of population as the survey effort varies from year to year. Estimates of population density (units not important for this illustration) oscillate substantially but there is no evidence of an overall decline in barbel densities. Otters are not getting a fair press.
Below is a count of barbel from the 1991 to 2018 EA surveys on the same section of the River Severn. No clear evidence of decline here either, the converse if anything, but the usual warnings about uneven sampling apply.