MARINE SCIENCE ASSESSMENT OF CAPTURE-BASED TUNA (Thunnus orientalis) AQUACULTURE IN THE ENSENADA REGION OF NORTHERN BAJA CALIFORNIA, MEXICO

Northern Bluefin Tuna (NBT, Thunnus orientalis) are found in the Eastern Pacific Ocean (EPO) from the Gulf of Alaska to southernBaja California and in the Western Pacific Ocean (WPO) from the Sakhalin Islands to the northern Philippines. They are usually oceanic but seasonally come close to shore, school by size, and tolerate a wide range of temperatures. They spawn in the WPO in the vicinity of Okinawa, Japan and the Philippine archipelago, then disperse to other areas of the WPO. Some fish apparently remain their entire lives in the WPO, while others migrate to the EPO during their first and second years of life. Fish in the EPO have an increasingly restricted north-south distribution as they grow older. Migrations between and within the WPO and EPO are related to oceanographic and prey conditions. Fish migrate back to the WPO between ages 2-3. During El Niño events, NBT are distributed further to the north in the EPO and catches decrease. Large impacts related to changes associated with global warming may limit the amount of NBT available off Baja California. Japan currently accounts for about 64% of the catch of NBT in the North Pacific Ocean (NPO). The other two nations involved in this fishery to a significant degree are Taiwan and Mexico. Catches historically have been 2-3 times higher in the WPO than in the EPO. The catch in the EPO in 2006 was ~10,000 metric tons (MT). Most of the catches in the EPO are fish of ages 1-3. Modeling studies have shown that a strong recruitment event occurred in 2001 and couldmaintain NBT spawning stock biomass until ~2010. The results of yield-per-recruit and cohort analyses indicate that greater catches in the NPO could be obtained if the catches of ages 0 and 1 fish were reduced or eliminated, mainly in the WPO. Increased fishing pressure on NBT juveniles from CBTA would not necessarily decrease recruitment, since spawner-recruit analyses indicate that the recruitment of NBT would not necessarily increase by permitting more fish to spawn. Even though fishing mortality (F) has been higher than FMAX,, or is above the reference point, recruitment overfishing has not occurred. Nevertheless, itis recommended that fishing mortality not be further increased and catches reduced. According to international institutions (IATTC, ISC and FAO), NBT is “Fully Exploited”. NBT is notincluded on the IUCN red list. There is no scientific evidence thatNBT are overfished in the NPO. Estimated retained catches of NBT have fluctuated widely between 500 and 10,000 MT in the EPO over the last 30 years (from 1976 to 2006). Regarding the CBTA activity, there is no evidence that it has affected the NBT stock since its beginning in 1996. Considering that not all NBT migrate to the EPO, increasing the catch of NBT would not necessarily decrease recruitment. Current CBTA productionlevels do not appear to compromise the NBT stock. However, catches of NBT juveniles and fishing effort should be regulated and not be increased, both in the WPO and EPO. Most of the capture-based tuna aquaculture (CBTA) facilities in Baja California use fresh, locally-caught Pacific sardine (Sardinops sagax caerulea) as feeds. Pacific sardines are oviparous, multiple-batch spawners that can reach a maximum size of 41 cm with a life span of 14 years. Fecundity is size and age dependent. Older fish spawn more times during a year, with spawning dependent onwater temperatures. Most recent stock assessments show that the stock productivity ofPacific sardines (recruits, age-0 fish, per spawning biomass) is declining, with stock spawning biomass (age +1) leveling off at 1.06 million metric tons (MT) in 2005. Studies suggest that the equilibrium of the spawning stock biomass and potential sustainable yield are dependent on environmental conditions. Recruitment success isvariable in long, decadal, time scales, depending on oceanographic conditions. There are three stocks of Pacific sardines. The sardine fishery based in Ensenada is the northernstock of this species. This stock is also fished by the USA and Canada. Other solely Mexican sardine fisheries comprise the southern and Gulf of California stocks. The fishery in Ensenada has traditionally been based on catches of small Pacific sardines of the northern stock. There are a high proportion of juveniles in these catches, since Ensenada vessels operate close to the coast (less than 40 nautical miles). Studies suggest that older and larger sardines move offshore where little fishing effort is currently occurring. Traditionally, the Mexican catch has been used for reduction to fishmeal and oil, canned for human consumption, or used fresh for bait. Landings of Pacific sardines at Ensenada increased from an annual average of 2,133 MT during the 1980s, to an average of ~48,000 MT in the 1990s. Landings decreased to ~41,000 MT during 2003 and 2004 and rose to 57,000 MT in 2006. Management of the Pacific sardine fishery inMexico incorporates several measures, including minimal sizes, closed seasons, and moratoriums on efforts. Fresh Pacific sardines have become important resources for CBTA in Mexico. In 2006, ~53% of the Pacific sardines landed in Ensenada were used for CBTA. However, this is likely an underestimate. Some catches were not recorded when they were delivered directly to the CBTA cages. Recently, a new Baja California State sardine fisheries committee that included fishing and frozen fish processing companies was reorganized to include CBTA in the management of the Mexican portion of the northern Pacific sardine stock. CBTA effects the marine environment and marine species associated with farm sites (sea mammals, marine birds, and marine organisms that inhabit the water column and benthos). Unconsumed or macerated sardines and fish feces are the main sources of solid, suspended, and soluble wastes. However, this waste stream is quite different from other, commercially fed finfish aquaculture such as salmon: tuna farming is seasonal; does not use antibiotics, chemicals, or any agricultural pesticides or additives; and it depends on natural feeds (sardines). Soluble nutrients are commonly detected only in close proximity to the tuna cages and dissipaterapidly. However, changes in the benthic community derived from enrichment of waste organic matter to the sediments could be more persistent, and even a 6 month fallow period may not be sufficient for the benthic community to recover. The extent of the increase in benthic enrichment is still to be determined since accumulations of tuna farming wastes are strongly dependent on the hydrodynamics and oceanographic characteristics of the farm site, and farm management practices, and no such studies have been done. Non-lethal methods of controlling marine mammals, such as placing high nets and electrified wires around the cages have effectively discouraged sea lions. The non-lethal use of whips and sounds to reduce bird predation on sardines are additional, successful measuresthat ranchers have implemented to avoid conflicts with protected species. CBTA has a number of environmental, social and economic impacts which can be considered as positive or negative, as judged by society. CBTA is a new economic activity within the fisheries sector of Baja California, México which has brought new jobs. CBTA is closely monitored by the federal authorities, and the management is carried by the proper government agencies. A new Mexican law onsustainable fisheries and aquaculture addresses CBTA. All CBTA farms in Mexico are required by law to monitor marine water quality and sediments; monitoring programs must be verified by both the Mexican Navy and the Ministry of the Environment. To date, neither agency has declared any negative environmental action on any of the tuna farms. Governance of tuna ranching in Mexico is still underdeveloped; several issues need to be addressed in order to assure a minimum impact on the environment, especially in regards to better scientific determinations of the carrying capacity of each site, and development of better technological and management alternatives to reduce the impact of fish wastes on the benthos.