Mesopelagic micronekton are a diverse assemblage of small (~2–20 cm) fishes, shrimps, and squids (see composite image below). Mesopelagic means that they inhabit deeper water during the day, staying in relative darkness, and ascend towards the surface at night when the risk of being seen and eaten by predators is less. These animals consume zooplankton and are in turn are food for commercially important pelagic fishes including albacore tuna, bigeye tuna, and swordfish. In collaboration with scientists at the National Marine Fisheries Service - Pacific Islands Fisheries Science Center we have been investigating the distribution and community composition of mesopelagic micronekton in the Pacific and the factors affecting it.
Our findings are briefly outlined below.
Cross Seamount - Within Hawaiian waters the capture of tunas and billfishes is not evenly distributed—some locations yield higher catch rates than others. Cross Seamount, located to the southwest of the main Hawaiian Islands, is a site of enhanced bigeye tuna catch-per-unit effort. The longstanding general belief has been that the seamount somehow generates greater abundances of micronekton forage for the deep-diving tunas. Bigeye tuna caught at Cross Seamount are found to have fuller stomachs than those caught at other Hawaiian Island locations, lending support to this hypothesis. We have used trawls and acoustic techniques to study the influence of Cross seamount on mesopelagic micronekton. Briefly we have found that micronekton are less abundant over the seamount compared to stations far from it. Most of the reduction is the result of strongly migrating species suggesting that they actively avoid the summit of the seamount although absence due to predation can not be entirely ruled out. Interestingly two species, a squid and a fish, were found in higher abundance over Cross and are considered components of a unique seamount community. See De Forest and Drazen 2009 in publications.
Hawaiian Islands - Regional comparison of trawl samples suggests that the lee of the Hawaiian Islands have a two fold higher abundance and biomass of micronekton. This could be through enhanced primary production through island runoff known as the island mass effect or it could be the result of eddy activity which also enhances primary production (although transiently) in the lee of the Islands. Ssee Drazen and De Forest 2008 in publications.
American Samoa - We are in the process of describing the mesopelagic micronekton community of American Samoa. This regions micronekton is very poorly described yet hosts important commericial fisheries.
Cross validation with acoustics - Acoustic surveys were conducted concurrently with all of the trawl samples taken. In conjuction with Reka Domokos at NMFS-PIFSC, we will be comparing our data sets in an effort to "ground truth" the acoustics.
Above - Sorting a trawl sample aboard the NOAA vessel Oscar Elton Sette
Below - A great diversity of micronekton from a tow between 400-600m at night in the open ocean.
Funding for this work has been provided by the Pelagic Fisheries Research Program
