Despite strong interests in developing offshore finfish aquaculture in California, no offshore farms exist. There is a catch-22 at work: concerns related to potential environmental impacts have slowed development of an offshore finfish industry, yet it has been difficult to define and address the real impacts of finfish aquaculture given the lack of farms in the state. To address the potential impacts and move forward in developing an aquaculture industry, the state needs better information about the actual impacts — both positive and negative — of specific farming practices in similar locations.
This project is assessing the impact of a tuna ranch just south of the US-Mexico border on wild seabirds and nearby island ecosystems. Seabird nesting links marine and terrestrial ecosystems and is therefore a useful marker for potential ecosystem impacts of offshore finfish farms. The researchers are comparing how the tuna ranch has affected the coastal ecosystems of the nearby Coronado Islands, using Cabrillo National Monument — a nearby site lacking significant densities of nesting seabirds — as a reference. The team hypothesized that the presence of the tuna ranch brings nutrients onto the island, as seabirds feed on bait fish and deposit guano, which can positively impact plant growth. This could mean that the presence of finfish aquaculture near degraded island ecosystems, especially islands impacted by introduced predator species, could jumpstart restoration efforts.
Observations of the tuna pens, along with information from Baja AquaFarms revealed that flocks of mostly western gulls and brown pelicans consume significant amounts of baitfish fed to the tuna. By analysis of the spatiotemporal variation in seabird nesting across island sites did not find decreased nesting in more remote islands. Factors besides proximity to the tuna pens appear to be responsible for the differences between sites. Stable isotope measurements from tissue collections, both from sites and from archives, are currently being analyzed.