Black Marlin

LIFE HISTORY

Figure 3: Size (Kg) distribution of fish taken in the Lizard Island - Cairns region by recreational anglers.

MATURITY

NW Coral Sea

Examination of the gonads of 74 female black marlin and 541 males taken in the longline fishery indicates that males reach maturity at about 30kg and females at about 100kg. Ovaries exceeding 1.0kg in weight are most certainly mature.

Australian East Coast

Gonad weights expressed as a percentage of whole body weight provide a good indication of the sexual status of fish. Examination of this index for fish taken in the recreational fishery indicates that males are not mature until at least 50-60kg. The missing size classes in the East Coast fishery include those females which have recently matured and therefore size of sexual mature females has not been determined.

Gulf of Thailand

Examination of 77 fish under 90kg weight showed that for individuals with gonad weights of <1kg, eggs in the ovary were discernible, but opaque, and testes could not produce milt under pressure. Few males and females of around 80kg were mature.

Eastern Pacific

The gonad weights of fish taken east of 130^oW clearly suggest that while black marlin in the Eastern Pacific are large enough to be mature, there is no evidence of any extensive spawning activity. Females weighing between 30 and 350kg rarely have gonad weights exceeding 1.5kg and a Gonad Index of 1.0

AGE and GROWTH

Methods of ageing marlin, other than size-frequency, have yet to be extensively explored or validated. Several potentially useful strategies are available with each relying on the assumption that age is not necessarily linked to somatic (approximately equivalent to whole body) growth. Earstones (otoliths), fin spines and vertebrae each display growth rings which may represent daily and yearly growth periods. Otoliths may also exhibit external ridges which have been suggested as annual in their frequency. Counts of these ridges for five black marlin between 67 and 248kg have returned figures of 6 to 18 (years?).

Tetracycline mark ~6 months inside the margin of a fin spine of a Black Marlin.

Indian Ocean

Age based on presumed annuli in a combination of internal and external features of the sagittal otolith have provided the following estimates.

Males: 75-105kg (4 fish) 5 - 13 years
Females: 97-275kg (17 fish) up to 20 years

East Coast Australia

The following presumed age-weight relationships have been determined for eastern Australian fish based on weight-frequency distributions compiled from Fisheries Research Institute (FRI) yellow tag returns.

Presumed Age	Modal Wt (kg)
1+	15 - 20
2+	30
3+	50 - 55
4+	70

Evaluation of growth increments in the otoliths and fin spines of fish taken in Queensland waters employed a technique of marking which provided confirmation of yearly growth increments in the fin spines of black marlin. This technique, utilizing oxytetracycline hydrochloride (OTC or "tetracycline") to mark the bone, was perfected by researchers at AIMS and principally carried out by professional charter boat crews operating out of Townsville on the Cape Bowling Green billfish grounds. Ages estimated from this work were:

Age	Wt (kg)
0+	9.0*- 13.6
1+	18.2 - 22.7
2+	29.6 - 35.6
3+	32.9 - 55.9
4+	77.5
5+	91.5
6+	154.0
7 - 13	313 - 424

* - smallest fish examined

Age estimates for 1+ to 4+ fish agree quite closely with ages derived from weight-frequencies (figure 4) and highlight the rapid growth rate of this fish. Furthermore, amongst juvenile black marlin, both males and females grow at similar rates. The continued growth of females, well beyond the maximum size of males, most likely occurs following sexual maturity of the fish.

Figure 4: Weight frequencies

Figure 5: Age of Black Marlin relative to weight and body length based on growth rings in the 3rd anal (A3) and 3rd dorsal (D3) fin spines.

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Last updated - 28 October 98

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