Estimation of return migration swimming speed of the Sirajo Goby, a Caribbean diadromous fish
The Sirajo Goby (Sicydium spp.) is an amphidromous fish species crucial to the estuarine ecology of Puerto Rican rivers. Amphidromous describes the unique life cycle of fish that live and spawn in fresh water; its pelagic larvae drift downriver and out to the ocean; the post-larval fish migrate through estuaries during the third-quarter moon phase to mature in freshwater rivers. We studied the Sirajo Goby to ultimately quantify the number of fish recruited during migration. We wanted to determine swimming speeds of post-larvae to characterize the rate of return to the Arecibo River, Puerto Rico, in constant water velocities, as well as the rate of upstream migration in open water velocities. A Go Pro digital camera recorded in-situ swimming of the Sirajo Goby migration at two locations: at the river mouth to determine the rate of return to the Arecibo River, and at an upstream riffle to determine the rate of upstream migration. The camera recorded video at 30 frames per second, which allowed us to play back video frame-by-frame. We measured body lengths of fish from the video and used these data to determine swimming speeds as the relative measure of body lengths per second (BL/s).
We found the Sirajo Goby migration was facilitated by the estuarine ecosystem of the Arecibo River. As fish migrate from the ocean to the Arecibo River mouth in approximately 0 m/s water velocity, swimming speed ranged from 1.9 to 29.2 BL/s. The environmental conditions allowed fish to recruit easily en masse into the river mouth. As the fish migrate upriver, however, increasing water velocity has a limiting effect on their upstream movement. Swimming speed did not exceed 5 BL/s at water velocities over 0.13 m/s. We will estimate the numbers of fish recruiting to Arecibo River to make recommendations regarding their management to state and federal resource agencies. Return rates to freshwater will be paired with instantaneous abundance measurements to estimate the total number of fish recruited. These post larval fish are an important nutrient source for larger game and nongame fish in the ecosystem. The Sirajo Goby is also an important source of income and cultural pride in the city of Arecibo and other coastal communities. Considering the limiting influence of increasing water velocities, the management of stream discharge may be important to the recruitment success of Sirajo Goby in upstream habitats.