Anglers keep a line on black marlin

A 200 pound black marlin off the coast of Hawaii. Photo / Getty Images

Divers and fishers help understanding of climate change on key fish species.

We know that climate change is driving changes in the world's oceans. Currents are shifting, temperatures are climbing and the availability and dynamics of nutrient upwelling is changing.

But the question is whether marine species can adapt at the rate at which these changes are occurring.

The coastal waters of south-eastern Australia are a climate change hotspot, warming at a rate three to four times the global average.

This is in part because of an increase in the strength and southward penetration of the East Australian Current (EAC), which carries warm water from the tropics down Australia's east coast.

In response, numerous marine species have been documented extending their distributions polewards, affecting the functioning of coastal and marine ecosystems in southeastern Australia.

This will have knock-on effects for local communities and fisheries, many of which are not well prepared.

With so many species on the move and changes happening so quickly, scientists have enlisted the help of citizen scientists - such as recreational scuba divers and fishers - to help record when, where and how often species are sighted.

Another successful example of citizen science is the New South Wales state government's game fish tagging programme. This world-leading game fish-tagging programme, established in 1974, asks recreational anglers to tag and release game fish and provide information on the species, size and release location which is sent back to the Department of Primary Industries (DPI).

More than 400,000 fish from at least 20 different species have been tagged, and more than 7000 recaptures recorded.

This has enabled us to investigate whether there had been any geographical shifts in suitable habitat for the highly mobile black marlin (Istiopmax indica) in the previous 16 years.

The black marlin is one of the most keenly sought game fish species targeted by recreational anglers in Australia, with more than 54,000 records of tagged black marlin within the NSW DPI's database.

An annual aggregation of large adults, some weighing more than 500kg, occurs off the northern Great Barrier Reef each spring, forming the basis of a charter fishery that will celebrate its 50th year of operation next year.

At the other end of the spectrum, juvenile black marlin from 15kg to 40kg undertake an annual migration southward along the east coast in association with the EAC.

The extensive spatial and temporal coverage of the tagging data allowed us to model the geographic distribution of black marlin habitat in the Southwest Pacific for 192 consecutive months from 1998 to 2013.

We found variability in the location of suitable black marlin habitat across months and years.

On an annual basis, conditions favoured by black marlin occurred off north Queensland at the start of spring and gradually shifted south along Australia's east coast from October to April.

This coincided with the peak availability of black marlin to recreational anglers and also to a seasonal pulse in the EAC.

In addition to the large variability on shorter time scales, we also found that suitable marlin habitat has shifted south at a rate of about 88km a decade across all seasons, independently of the influence of ENSO.

We found that habitat is shifting faster during summer months (111km a decade) in contrast to the rest of the year (77km a decade).

This suggests that suitable habitat is extending south quicker than it is contracting at its northern edge.

What is clear from this study is that mobile fish species are not immune from the impacts of climate change, and that long-term data sets from recreational fishers are valuable tools in discerning such changes.

• Marine biologist Dr Julian Pepperell and James Cook University student Nick Hill co-wrote this article.