Nigel
When otter groups were at their peak of reintroductions (about 8 years ago) there was no thought or ecological assessment done to see whether the habitat, which they were to be released into would support them.
This perhaps was naive by these groups to think; large area of open landscape with what appeared to be plentiful waterways equalled otter survival.
It is thanks principally to Chris Burt and the SAA that the Otter Trust and otter groups no longer release captive bred otters into the wild. I also know that this was one of the main arguments that Chris used to persuade them to stop such releases.
IN SHORT, CAPTIVE BRED OTTERS ARE NO LONGER RELEASED.
Eels whilst being the favoured food of otter, and sadly are in decline, have characteristic reasons as to why they are. Eels unlike other fish when disturbed flee to the nearest refuge (rock, tree, etc) rather than take flight and swim off at a rate of knots and keep going until they?re out of danger. This makes them easier prey to catch due to the dexterity of the otter?s paws, claws and sharp teeth, therefore making them a favoured food.
Innately and subconscious all predators make a judgement call about energy balance. How much energy will I need to catch my prey if it moves fast, as opposed to catching something slower but less plentiful in fish biomass terms.
Baz Mark is quite right to turn what you said upside down. All top line predators need huge territories and otters are no different. In one of Nikko Tinbergen?s books he devotes a chapter to discussing why big scary animals are rare. Otters are in European aquatic terms the biggest scary animal of all.
Lets be clear here otters because of their natural rarity (big scary animal syndrome) will never be plentiful and all talk of equating them to cormorants is just plain wrong.
Over population of big scary animals leads very quickly to a crash in the population due to not enough food supply.
Baz in the whole of the Ribble catchment area there are no more than 6 animals, which, on average eat about the same amount of fish per day as one cormorant 1.25 kilos of fish. God I wish there were only 6 of the black death on that river.
Just as a side issue whales are the biggest animals on the planet (debatable whether they?re scary), and there are many species of them, far more that there are big cats, and if what I?ve wrote above is true, why?