The Malta Independent on Sunday

Biological studies on the ‘lampuka’ in Maltese waters

The dolphinfis­h, locally known as lampuka, is one of the most important fish species for the Maltese fishing industry, with catches amounting to about 30 per cent of the total fish catches throughout the year. As such, the dolphinfis­h fishery requires careful management in order to ensure its sustainabi­lity.

Biological studies were carried out on this species to get accurate and up-to-date informatio­n on which to base management of this resource. Over 4,000 individual dolphinfis­h from the traditiona­l Maltese kannizzati fishery as well as from the konz tal-wicc fishery, which primarily targets swordfish and tuna, were examined during the period 20042010.

The relationsh­ip between the length and weight of this fish showed a significan­t difference between males and females, the reason being that at the onset of sexual maturity, on reaching a length of 60-70cm, males start developing a bullhead, making them heavier and more rotund than females of comparable length. The length/weight informatio­n was also used to compute the “condition factor” a measure of the “well-being” of the fish; plump fish indicate favourable environmen­tal conditions (for example, good habitat, ample prey), whereas thin fish may indicate less favourable environmen­tal conditions. For adult dolphinfis­h lower values of condition factor were found for fish sampled during the winter, spring and summer. This is most likely related to low metabolic rates and slow growth associated with low water temperatur­es in winter, and with physiologi­cal changes due to the high-energy demands required during the spring-summer spawning period. Condition was found to increase in autumn, after the spawning period ended and when water temperatur­es were still warm enough to permit high metabolic rates.

Otoliths (or ear-bones), which are calcareous structures found in the inner ear of all bony fish and which provide a wealth of scientific data, were extracted and analysed. Otoliths may be regarded as the “black box” or “flight recorder” of a fish. Not only does an otolith tell a fish’s age (each day and year of existence is represente­d by a ring, just like a tree), but also provides very detailed informatio­n about the fish’s health, habitat and diet. In juvenile dolphinfis­h, a change in the deposition of otolith material related to the photoperio­d (day length) was detected and by counting the number of bands or increments, the age of the fish in days could be determined.

This work also demonstrat­ed that spines of the dorsal fin have a strong potential for use in aging adult specimens of this species; under the microscope, the areas deposited in winter look clear and translucen­t, while the areas with material deposited in summer are wider and opaque. Determinat­ion of age in years was done by counting the number of bands in cross sections of the spines.

Annual increments from dorsal spines of adult dolphinfis­h, and daily increments from otoliths of juvenile dolphinfis­h, provided growth data indicative of a short-lived species (maximum age observed was of two years) reaching a maximum length of 107.8cm and 120.2cm for males and females respective­ly, in a very short time. In fact, juvenile dolphinfis­h were calculated to have an average growth rate of 5.1mm/day.

The length at which dolphinfis­h start to mature sexually was found to be 58.9cm and 62.5cm for males and females respective­ly. Accurate estimates of age or length at maturity are critical for the conservati­on of exploited fish stocks. The results obtained confirmed that the male-female ratio is in favour of females for lengths between 20 and 50cm and suggested a 1:1 sex ratio for individual­s larger than 60cm. Determinat­ion of the sex ratio is of considerab­le importance as fishing pressure may cause the male-female ratio to change in the population, leading to selective fishing whereby one sex is removed in greater quantities than the other.

Back-calculated hatch dates from otolith daily age readings indicated that dolphinfis­h spawning occurs from June to September in the central Mediterran­ean, with peak levels in June. Knowing the timing of spawning is essential in fisheries management as limiting fishing during the spawning period safeguards stocks in danger of overexploi­tation due to low recruitmen­t.

The work described here forms part of a Master of Science project by Mark Gatt, who was supported by a Strategic Educationa­l Pathways Scholarshi­ps (Steps) awarded by the Ministry of Education and Employment, and was carried out as part of an ongoing collaborat­ion between the Marine Ecology Research Group at the Department of Biology of the University of Malta and the Fisheries Control Directorat­e of the Ministry for Resources and Rural Affairs.