Pelagic fish species such as tunas and billfishes are highly mobile and often undertake long distance migrations. Tagging studies have shown that billfishes and tunas have annual migrations that can traverse ocean basins and tens of thousands of kilometers to reach spawning and/or foraging grounds. These species are targeted and caught as bycatch in numerous longline fisheries and recreational fisheries around the world
As these fish are epipelagic, upper tropic level predators, they also play important roles in maintaining the health and function of marine ecosystems. Quantitatively characterizing the distributions and habitat use of these top predators and understanding their potential responses to climate change is therefore vital for timely and effective management and conservation decisions.
Blue marlin (Makaira nigricans) are a large, highly migratory species of billfish (family Istiophoridae) distributed globally in tropical and subtropical waters.
Although known to spawn in several locations, such as Japan, Hawaii, French Polynesia, the Straits of Florida and Bahamas, the drivers of blue marlin movements are still poorly understood. Blue marlin are frequently caught as bycatch in tuna longline fisheries but also targeted in artisanal and recreational fisheries. The combined catches of these fisheries have led to a listing of vulnerable by the International Union for Conservation and Nature.
Blue marlin are considered a single global species with separate stocks in the Atlantic, Pacific and Indian Oceans, although information on blue marlin in the Indian Ocean is limited. However, recent genetic evidence suggests blue marlin in the Pacific and eastern Indian Oceans constitute a single stock Acoustic and satellite studies indicate that blue marlin are epipelagic with a bimodal depth distribution.
Daytime habitat is primarily between 80 feet and 325 feet with periodic excursions into deeper water, whereas nighttime habitat is generally within the top 35 feet of the water column. Sea surface temperature (SST) and dissolved oxygen are thought to be significant drivers of habitat use for blue marlin.
This species is generally found at temperatures above 24 degrees Celsius, but it prefers warmer waters up to 30 degrees. Shoaling of the oxygen minimum zone in the Eastern Tropical Pacific (ETP) and Atlantic (ETA) are thought to limit the vertical habitat of blue marlin.
Low temperature and low dissolved oxygen have a similar effect when they co-occur in the Pacific.
Environmental variability related to the El Niño Southern Oscillation (ENSO) cycle is also thought to influence the habitat use of blue marlin in the Pacific. It found that the distribution of blue marlin shifted eastwards during the 1997-98 El Niño associated with increasing SSTs and a deepening of the thermocline. This suggested the westward extension of cold, low-oxygen water into the Central Pacific associated with the 2010 La Niña may act as a barrier to trans-equatorial migrations into the South Pacific. Studies suggest that blue marlin distribution is strongly influenced by the physical environment.
Species distribution models (SDMs) are important tools that are increasingly being used to quantify the spatial distribution of species habitat in terms of environmental conditions.
Previous studies have used SDMs to evaluate the effect of environmental variables and seasonal variation on blue marlin distribution and abundance in the Pacific using longline fisheries data.
In this study, the habitat preference of blue marlin is evaluated, and its distribution is predicted from 2000 to 2016, throughout their worldwide range using a unique, long-term telemetry data set, collected mostly through the International Game Fish Association Great Marlin Race (IGMR).
Seasonal variability of blue marlin habitat and the potential effects that climate variability may have had on its distribution is also being investigated.
ACKNOWLEDGEMENTS: Jonathan J. Dale, Stephanie Brodie, Aaron B. Carlisle, Michael Castleton, Elliott L. Hazen, Steven J. Bograd, Barbara A. Block, The Great Marlin Race with HIBT, IGFA, Moore Foundation and Stanford University.