The lowdown on dolphin fish

As the warm ocean waters finally cool, the East Australian Current retreats, the network of Fish Aggregation Device (FADs) is retrieved in places like NSW, as the game fishing begins to switch to other species, it's well worth taking stock of the magnificent mahi mahi aka dolphinfish or dorado.

Thanks to angler citizen science and the rec fish tagging, some amazing information is coming to hand about this fast living pelagic fish in Australian waters.

Fishing for mahi mahi is becoming an increasingly popular activity for recreational anglers around Australia. The continued expansion of the FAD network in NSW has created many opportunities to target the species, however, little is known about their movements and behavior off eastern Australia.

To solve some of the mysteries behind mahi mahi, scientists have spent the past four years researching the ocean habitats of the wide-ranging species and their interactions with FADs off the NSW coast.

ACOUSTIC TAGGING
Acoustic tags were used to monitor 38 mahi mahi in a network of FADs deployed off NSW. These electronic devices record when a tagged mahi mahi is swimming within a few hundred meters of a FAD. Mahi mahi were found to spend only short periods of times in the proximity of individual FADs, with most fish aggregated for a period of two days prior to departing the FAD.

Seven of the tagged mahi mahi exhibited movements to other FADs in the network. One mahi mahi in particular was tagged at the Ballina FAD, detected at the Port Macquarie FAD one month later, and detected again at Terrigal after an additional six days. This individual swam at least 550km in this time period.

Another mahi mahi reacted strongly to being tagged at Port Macquarie, and swam to the Laurieton FAD 30km away in under two hours! Connectivity between FADs, and the short residence times, suggest that local depletion of fish stocks at FADs is unlikely in NSW.

GAMEFISH TAGGING PROGRAM
Researchers also examined the ocean habitats of mahi mahi using 19 years of tag-and-release records from the NSW DPI gamefish tagging program.

Mahi mahi were found to exhibit strong seasonal migrations into NSW water every summer and autumn, with fish most abundant in waters 24°C or above. Ocean fronts were preferred by mahi mahi; such oceanographic features can enhance feeding opportunities for pelagic fish.

The next step for researchers is to use this information to better predict the influence of changing ocean currents on the distribution of mahi mahi.

Researchers also hope to use this information to promote dynamic FAD deployments that better respond to variability in mahi mahi abundance.

This work represents a collaboration between the University of NSW, NSW Department of Primary Industries – Fisheries, and CSIRO, and funded by the NSW Recreational Fishing Saltwater Trust, the Australian Research Council, and CSIRO.

Photo credits: Opening photo by Craig McGill of Fishabout Charters; underwater photo by Shane Chalker Photography.