Tuesday, 6 December 2011

Sticklebacks glisten

For the first time in several years of trapping Crayfish on this particular stretch of the River Lark, I have caught a sizeable haul of Sticklebacks and Minnows. While I might normally catch the odd individual here or there, the two smaller-meshed traps positively glisten with tiny fish this morning. Such small fish would be no match for the Signal Crayfish so it is fortunate that there are few of these voracious crustaceans in the traps today. I do not know why there should be quite so many fish this morning but seeing them here in such numbers brings back memories of childhood.

Like many budding naturalists I had my first experience of sticklebacks as a child, pond-dipping in some old mill ponds at the bottom of the hill below our house. I then read about them in biology text books, kept some in a tank and watched enthralled as the males built their delicate nests and wooed females with their bright red bellies and construction skills. It is perhaps unsurprising that the stickleback should be one of the most well studied fish in the world. One of the neat things about them is (in most stickleback species) the ability to live equally successfully in freshwater and the sea, two very different environments presenting different challenges.

Sticklebacks are small fish, typically some three to seven centimetres in length, with a series of short spines along the back from which they derive their name. The spines are modified fin rays; those on the back are erect and obvious, while those on the side less so and apt to catch out the unwary child, removing a prize fish from a shrimping net. The lack of true scales gives these little fish a rather soft-skinned appearance. They may carry a series of bony plates along their flank, although the extent of these may vary between individual fish, a useful feature by which individuals can be identified within the confines of a cold-water aquarium.

In weedy waters, Three-spined Sticklebacks tend to be solitary in their habits but in more open water, such as this stretch of the River Lark, they shoal. Shoaling provides safety in numbers, with more eyes alert to predators and an individual’s risk of being predated falling as the size of the shoal increases. Minnows often form part of these mixed shoals, so maybe I have just got lucky and attracted a shoal into the traps. Sticklebacks are visual hunters, with well-developed eyes, feeding on small invertebrates. Although short-lived (most will spawn only once), they are a successful species with a wide distribution. This, coupled with the relative ease by which they can be caught, may explain their attraction for children and researchers alike.

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