AIMS Research - Black Marlin EARLY LIFE HISTORY

The gonads of a female black marlin in spawning condition may weigh as much as 30kg and represent 5% of whole body weight. A fully developed egg is approximately 1mm in diameter. Egg counts suggest that a large mature female may have between 67 and 226 million eggs.

Larvae of black marlin have rarely been collected in oceanic waters despite extensive surveys. This may partly be due to black marlin larvae being found closer to coral reefs and shelf waters than generally sampled by these surveys. It may also be partly due to identification problems which have never been satisfactorily resolved. Recent studies off the Ribbon Reefs of the GBR collected significant numbers of larval black marlin concentrated within 0.25 miles of the coral reef crests. Larvae were taken through October and November, with only a few taken in January-February and none in March.

The occurrence of 1+ fish and older has been discussed above. Fish of less than 10kg (0+) are only rarely caught. This may either reflect a catchability problem in both recreational and commercial fisheries or, given that occasional small fish are caught by anglers on the continental shelf, may be due to their occurrence predominantly in oceanic waters where they are too small to be caught on longlines. Post-larval fish to around 10cm infrequently, but regularly, turn up in the stomachs of pelagics such as dolphin fish and tuna species. This prompts fishermen to believe that spawning has taken place nearby but, as suggested above, these early life stages may be oceanic and subject to considerable displacement by oceanic currents, for example the East Australian Current with a strong southerly set.

Stomach contents of 10-40kg fish in nearshore waters of North Queensland are dominated by the tropical pilchards (Amblygaster spp.) and herrings (Sardinella gibbosa). In southern waters, Sardinops neopilchardus, the southern pilchard, predominates.

Food item

count (%)

prevalence

Amblygaster sirm

Sardinella gibbosa

Decapterus spp.

Sarda sp.

Carangoides sp.

Chirocentrus dorab

Sphyraena sp.

empty

billfish remains *

132 (78)

25 (15)

4 ( 2)

3 ( 2)

1 ( 1)

- -

4 -

* - includes bills and mandibles from LI fish. Values are based on LI fish only.

33 species of parasite have been recorded from black marlin. Nineteen of these have only been recorded from Queensland coastal waters. The parasite fauna of black marlin strongly reflects their feeding habits. As an apex predator, they are the definitive host for a diverse assemblage of parasitic helminths and copepods. While some parasites appear to be specific for black marlin, several are found in, or on, other species of billfish.

The existence of at least three stocks has been suggested: South-West Pacific; Eastern Pacific and Indian Ocean (including East and South China Seas). There is a possibility of movements of fish between the Indian Ocean and South-West Pacific stock, either as a result of juveniles moving through Torres Strait or of fish swimming through the Banda Sea and around the northern coastline of Irian Jaya and PNG. No significant evidence is available to determine the extent of movement between the stocks, if any.

The existence of an isolated Eastern Pacific stock is highly unlikely. Despite relatively few black marlin having been tagged in the Eastern Pacific, a single tagged fish has moved from Baja California in the Eastern Pacific to the northern Tasman Sea in the South-West Pacific. In addition, studies of gonad development have found no evidence of significant spawning activity in the Eastern Pacific (see above).

The most important stock problem for management is the relationship between the NE Indian Ocean, East China and SW Pacific stocks.