When designing a research project, scientists must decide which gear and equipment are most appropriate for collecting the data required to address their hypotheses. One of the main targets of our project are juvenile fishes, primarily those living in Sargassum habitats, but also juveniles in nearby “open water” habitats (in other words, water without Sargassum). Often times a single project may require multiple samplers, and that’s the case with our Sargassum research.
Sargassum is a floating brown algae, therefore a sampler than can be ‘fished’ at the water’s surface would be ideal. That’s why many Sargassum studies often rely on a neuston net. Neuston is a term that refers to organisms that live on or just under the surface of the water. Our neuston sampler has a 2-m wide by 1-m high rectangular opening, and a plankton net with a 0.5-mm mesh size. To collect Sargassum or any neustonic organisms, the net is fished half-in/half-out of the water. In open water, we tow the neuston net for 10 minutes, where it collects mostly larval fishes and small juveniles (bigger juveniles are usually too fast and agile, and often avoid the net). In Sargassum, our tow times are much shorter (less than 1 minute) because the net quickly fills up with Sargassum. In contrast to sampling in open water, the neuston net does a great job collecting juvenile fishes in Sargassum, because these small fishes seek refuge in the algae as the net approaches. So the neuston net is an effective sampler in both habitats, but often collects different fish life stages–larvae in open water and juveniles in Sargassum.
In order to collect the larger juveniles that evade the neuston net in open water, this year we will try a new sampler designed to collect these elusive fishes. The Methot frame trawl is a much larger net. The mouth opening is a square shape, approximately 2.25-m high by 2.25-m wide. The mesh size is larger as well, approximately 3-mm, and the net can be towed at a relatively fast speed, around 5 knots. Our hope is that the larger net mouth opening and faster tow speed will help us collect the juvenile fishes that usually escape our neuston net in open water. This is our first time using this sampler, and we’re excited to see what we get!
To maximize our sampling time at sea, we try to sample at night as well as during the day, which presents a new set of challenges. The main challenge is that once the sun sets, it’s nearly impossible to search for and distinguish between Sargassum and open water habitats. So at night we need to use a different strategy and different sampler. As the sun begins to set, we place a floating marker buoy with a flashing light inside a Sargassum feature to “mark” the habitat. Then, we steam away from the Sargassum to open water, and wait for the darkness. Now it is light-trap time! We place two of these samplers in the water where they float for approximately a half hour at the surface. A light is used to attract fishes through small openings (approximately 2.5-cm in diameter) that lead to a mesh “trap” compartment. Once retrieved, we locate our marker buoy and steam back to the Sargassum and repeat our sampling there. Light-traps are selective samplers, in that they generally collect juvenile fishes that are photopositive (attracted to light).
In addition to these methods, we also float a camera system in the Sargassum habitat to video the “undisturbed” fish assemblages, and hook-and-line sample with small-hook Sabiki rigs to collect additional juveniles. By combining multiple gear types, we hope to better categorize the juvenile fish assemblages associated with these different offshore habitats.