Field observations on the reproductive biology of a newly discovered stickleback (Gasterosteus)

Field observations on a newly discovered form of stickleback, termed the white stickleback, were conducted at three sites in Bras d'Or Lake on Cape Breton Island, Nova Scotia. The white stickleback is sympatric with but reproductively isolated from the threespine stickleback (Gasterosteus aculeatus). White males nest above the substrate in filamentous algae, whereas threespine males nest on the substrate away from algae. At one of our sites, however, a few males of both sticklebacks were found nesting in similar habitat, namely on top of alga-covered rocks. Nevertheless, there was microhabitat separation of the two sticklebacks, because white males consistently nest farther from the shore than threespine males. Hence, there is little interaction between the two forms during courtship of females. In areas where territories were adjacent, white males attracted gravid threespine females to their nest, but courtship broke off at that point. Observations indicated positive assortative mating, as found in earlier laboratory work, and suggest that the two sticklebacks may be good biological species. However, the previously reported association between white sticklebacks and dense growth of filamentous algae is not perfect, since white males at one site had built their nests and were actively courting females before the filamentous algae bloomed. Some white males vacated their nest sites during the day, whereas threespine males never did. It is not clear why white males do this, where they go, or for how long, but the departures may be related to the high levels of activity shown by white males while on their territories.