By Amanda Croteau, UF PhD Student
The Original Light Trap
Meroplankton are animals that spend a portion of their lives (larval and early life stages) as plankton. These organisms eventually grow larger and become part of the nekton (animals that are able to swim and move independently of water currents) or benthic communities. Ichthyoplankton are the eggs and larvae of fish. Eggs are passive and dispersed by currents. Initially most larval fish have no or minimal swimming ability. As they develop, they become active swimmers.
It is important to study meroplankton and ichthyoplankton because they are indicators of the spawning population of adults, and the survival or mortality of meroplankton have a direct effect on adult population numbers. Species composition at a given location depends on the spatial distribution and reproductive habits (periodicity, fecundity, etc.) of adults, growth and larval stage duration, and abiotic factors that affect transport (currents, tides, salinity, etc.). Mortality depends on many factors such as predation, disease, food availability, and habitat. Habitat is important because individuals who fail to make it to their correct settlement or juvenile habitat are unlikely to survive. In estuarine environments, freshwater and tidal cycles play key roles in species distribution.
Mangroves and salt marshes provide vital juvenile habitat for many inshore, nearshore, and offshore marine species. Florida’s coastal habitats have been severely impacted by coastal development, and Tampa Bay has lost over 44% of its mangrove and salt marsh habitats (Lewis et al. 1985). Robinson Preserve is one of the largest (197 hectare) mangrove and salt marsh restoration efforts in Tampa Bay. Robinson Preserve was originally a coastal wetland that was ditched and drained in the 1920s for agricultural use. In 2006, tidal flow was restored through connections with Perico Bayou, Palma Sola Bay, and the Manatee River. Restoration also involved the planting of native upland and salt marsh vegetation. However, no efforts were made to supplement the aquatic flora and fauna, rather it was expected that they would colonize the preserve from neighboring populations. Ichthyoplankton and meroplankton abundances were selected as one metric to evaluate the quality of the restored ecosystem as nursery habitat.
Meroplankton and ichthyoplankton can be sampled in a variety of ways including light traps, benthic sleds, Miller high-speed samplers, push nets, tow nets, and light traps. As with any sampling gear, each method has its pros and cons and gear selection should be informed by target taxa, gear bias, and site constraints. Light traps utilize organisms’ natural attraction to light (photopositive) as bait. Photopositive taxa approach and enter the trap and are then funneled into a collection chamber. Light traps also allow you to sample continuously over an entire night at multiple locations. Robinson Preserve is shallow (generally <2 m), with complex habitat types and obstructions. It is also a no motor zone. Due to these study site constraints, light traps were selected as the most efficient gear to sample ichthyoplankton and meroplankton within the preserve. The light trap designed by Jones (2006) was redesigned for deployment from shore and scaled down for use in shallow, estuarine systems (Figure 1 and 2).
Among the larger organisms (≥3 mm) collected, 18 major taxonomic groups have been identified to date. Overall community composition was dominated by isopods (19%), caridean shrimp (18.2%), fish (15.7%), and parasitic copepods (13.1%), though species assemblages varied by site and season (Figure 3). The greater taxonomic richness in sites 1 and 3 is likely related to their locations. Both of these sites were located in areas with slower currents than in sites 2 and 4, which may have allowed some less mobile species to enter the light traps than in the latter two sites. Larval and settlement stage fish were collected in nearly every sample (Figure 4), including fish from at least 8 families. This is similar to the degree of diversity noted in other light trap studies in similar habitats (Hernandez and Shaw 2003; Strydom 2003). Juvenile mullet (likely Striped Mullet Mugil cephalus) were always present in winter samples, while juvenile clupeids (likely menhaden Brevoortia spp.) were present in the winter and spring, which corresponds well with their respective peak spawning periods within this region.
The use of a modified light trap in Robinson Preserve proved to be an effective method for sampling ichthyoplankton and meroplankton, as well as some other groups. Several fish parasites were collected in large numbers during this study. Juveniles of the parasitic isopod family Cymothidae were by far the most dominant form of isopod present in the light trap samples. Parasitic copepods of the genera Argulus and Caligus were also collected in large numbers. The high abundance of external fish parasites collected with this method may provide a new and efficient means of surveying such taxa in estuarine systems.
Hernandez, F.J., and R. F. Shaw. 2003. Comparison of plankton net and light trap methodologies for sampling larval and juvenile fishes at offshore petroleum platforms and a coastal jetty off Louisiana. American Fisheries Society Symposium 36: 15-38.
Jones, D.L. 2006. Design, construction, and use of a new light trap for sampling larval coral reef fishes. NOAA Technical Memorandum NMFS-SEFSC-544.
Lewis, R. R., R. G. Gilmore, Jr., D. W. Crewz, and W. E. Odum. 1985. Mangrove habitat and fishery resources of Florida. Pages 281-336 in W. Seaman, Jr., editor. Florida aquatic habitat and fishery resources. Florida Chapter, American Fisheries Society, Kissimmee, Florida.
Strydom, N.A. 2003. An assessment of habitat use by larval fishes in a warm temperate estuarine creek using light traps. Estuaries 26(5): 1310-1318.