By Emilie Stump
In part two of this three-part series, marine ecologist Emilie Stump reports on threats impacting seahorse habitats and populations in Biscayne National Park, with a focus on the relationship between land use and water quality and the commercial bait shrimp fishery.
In my search for seahorses and pipefishes to learn more about these cryptic animals I extensively surveyed habitats in the Biscayne National Park (BNP). The BNP is a living, breathing emerald and aquamarine jewel in the U.S. National Park System. Located in southeastern Florida, just south of “Magic City” (Miami, Florida), the park protects a 728km2 gradient of vulnerable coastal habitats including mangroves, seagrasses, and coral reefs. I also attended meetings held as part of NOAA’s Biscayne Bay Habitat Focus Area Initiative with representatives from federal, state, and local government, non-governmental organizations, academia, and representatives of the fishing industry to learn more about the history of the park, and current issues affecting it. These meetings and my on-the-ground experience revealed a park that is under pressure from a variety of threats.
How has land use and development affected Biscayne National Park?
The BNP protects a little bit of nature in a landscape otherwise heavily altered by humans. It is bordered to the north and the west by Miami-Dade County, home to nearly 2.8 million people. Urban and agricultural development has led to increased contamination of BNP by nutrients (like nitrogen or phosphorus), chemical pollutants, suspended sediment, and heavy metals from sources such as landfills, port dredging projects, agricultural or urban storm water run-off, and over-flowing septic tanks. Instead of being filtered through wetlands and mangrove forests, freshwater laced with contaminants is channeled into a system of man-made canals, which deliver concentrated freshwater discharges laced with a cocktail of contaminants in large pulses to Biscayne Bay. I especially noticed effects of these discharges along the western boundary of the park, near the mouths of the canals.
Decreased water quality has led to an increase in documented algae blooms in semi-enclosed, shallow water Biscayne Bay. These blooms block sunlight from reaching seagrass blades, leading to seagrass die-offs. Less seagrass may mean less available habitat for seahorses and other species that depend on them. Research from state and federal level management and monitoring agencies suggests that the frequency of algae blooms in the Bay is increasing, and that the Bay is in danger of reaching a “tipping point”, shifting from a seagrass-dominated system (photo on the left) to a system with substantially less seagrass regularly experiencing algae blooms in the water column and on the substrate (photo on the right).
Left: Healthy bed of Turtle Grass (Thalassia testudinum) in Biscayne Bay.Right: Seagrass bed covered with cyanobacteria in Biscayne Bay. Cyanobacteria blooms can result from excessive nutrients being introduced to the system.
Commercial bait shrimp trawling in Biscayne Bay
It surprised me to learn that the Biscayne Bay portion of BNP is subject to commercial fishing, including trawling for bait shrimp. Bait shrimp are harvested using a type of gear specific to Florida called the roller-frame trawl. According to a 1997 report by University of Miami researchers, roller-frame trawls causes minimal damage to seagrass beds but can cause substantial damage to potential seahorse holdfasts including corals, sponges, gorgonians and other organisms found in the hardbottom habitats of Biscayne Bay.
A fisheries-independent survey published by Dr. Joe Serafy and colleagues in 1997 revealed that roller-frame trawls are also known to catch at least eight species of seahorse or pipefish in BNP as bycatch. Additional research published by Dr. Julia Baum showed that large seahorses, such as the lined and long-snouted seahorses of BNP, may be the most vulnerable. This study estimated that 72,000 lined seahorses per year were caught as bycatch by 31 trawlers operating off Hernando Beach, Florida Possible effects on populations include direct mortality to seahorses and their relatives or disrupting social structure.
Dr. Baum, whose research took place in the Gulf of Mexico, documented that once these large seahorses were taken in as bycatch they were sometimes transferred to either the marine aquarium trade or the curio trade. During my time in south Florida I spoke with collectors for the marine aquarium trade who confirmed that they had received buckets of seahorses that were originally collected as bycatch from the bait shrimp fishery in Biscayne Bay. While collection of animals for the aquarium trade is regulated in Florida, animals entering the trade as bycatch may not be accounted for when setting yearly or seasonal catch quotas and are not likely to be reported in official catch statistics.
What does this mean for seahorses and their relatives in BNP?
Seahorses and their relatives are dependent on healthy habitats. Despite their protected status, habitats within BNP are exposed to a myriad of threats. Two potential threats are habitat degradation due to land-use practices, and habitat damage caused by bait shrimp trawling using roller-frame trawl gear. In addition to causing habitat damage, bait shrimp trawling disrupts populations and may illegally remove animals from Biscayne National Park for the curio and/or aquarium trades.
Part three will focus on conservation solutions by highlighting habitat-centered initiatives in the 2014 Biscayne National Park Fisheries Management Plan, and how implementing these measures will benefit seahorses and their habitats.