If asked, "what do spiders eat?", my answer would probably include insects, spiders, other arthropods, and maybe birds. I'd never have thought of fish!
And yet it seems that fish-eating by spiders is, if not common, then not exactly rare, although other food items still account for most of the spiders' diets. In a paper just published in PLoS ONE, Nyffeler & Pusey (2014) present evidence - from an extensive literature review - for eight-legged piscivores on every continent other than Antarctica, although they're more often found in tropical & sub-tropical regions. And it seems they're not alone: the authors list a number of other arthropods with similar tastes, including water scorpions, backswimmers, caddis flies and water boatmen.
The spiders involved were mostly from the genera Dolomedes & Nilus ie they are large (as spiders go: a big female Dolomedes can have a leg-span of 6–9 cm and weigh ~0.5–2 g) and semi-aquatic, spending a lot of time at the water's edge. Here's an image of a female Dolomedes from the UK, settling in to consume a stickleback:
Image: Nyffeler & Pusey (2014) doi:10.1371/journal.pone.0099459.g007
Incidentally, while we have spiders of this genus in New Zealand, it seems our small freshwater fish have little to worry about. Nyffeler & Pusey report that
only the largest of New Zealand's three species of Dolomedes (Dolomedes dondalei) was capable of catching fish in laboratory experiments whereas the two smaller species (Dolomedes aquaticus and Dolomedes minor) were not.
When hunting fish - & for most spiders the researchers note that fish are a relatively rare component of the diet - the arachnids seem to use touch (mechanoreception) rather than vision. They sit at the water's edge with their front pairs of legs spread out & resting on the water surface, and the others anchoring them to a rock or a plant. In some cases, especially when the water is calm, it seems that the spiders may detect their prey from ripples in the water, but in others their attack is triggered by the fish's dorsal fin actually contacting one of their legs. And while spiders usually eat other animals smaller than themselves, in the case of fishing spiders their prey may be more than twice as large as the predator, which means that there's quite a lot of effort involved in subduing dinner (usually done by biting the fish behind the head). and then dragging it out of the water to feed.
Nyffeler & Pusey cite experimental evidence showing that spider venom is quite capable of killing small fish, although it may take 20 minutes or more to do so. In the wild, that would be a long time to hang onto a wriggling fish. And why then drag it out of the water? Perhaps because the digestive enzymes injected into the prey would otherwise be diluted - remember that spiders are 'liquid feeders' who must wait until the prey's innards have been liquified by those enzymes before slurping up the resultant soup.
While the fish these spiders eat are a large prey item, & capturing them must incur some risk, the researchers argue that such hunting may well be advantageous at times when other prey items are rare. However, they conclude that
Complete piscivory is probably rare and restricted to those occasions when semi-aquatic spiders gain easy access to small fish kept at high density in artificial rearing ponds or aquaria or in small shallow waterbodies.
Owners of home aquaria and fish ponds may never view Dolomedes in quite the same way again...
Nyffeler M, Pusey BJ (2014) Fish Predation by Semi-Aquatic Spiders: A Global Pattern. PLoS ONE 9(6): e99459. doi:10.1371/journal.pone.009945