Introduction

Lights Have Often Been Used as aids in collecting various marine and freshwater organisms. These exploit the positive phototropism exhibited by certain species, e.g. cumaceans (Hale, 1953), aquatic insect larvae (Hungerford et al., 1955; Washino and Hokama, 1968), assorted freshwater invertebrates (Espinosa and Clark, 1972), marine invertebrates (Sheard, 1941) and fish larvae and juveniles (Winn and Miller, 1954; Parsons and Hodder, 1970). Various systems have been used, from suspended lights and dipnets (Winn and Miller, 1954) to more elaborate submersible and semi-submersible traps (Hungerford et al., 1955; Washino and Hokama, 1968; Espinosa and Clark, 1972).

The light-trap described here was designed for collecting larval and juvenile fish of the families Tripterygiidae (‘blennies’), Clinidae and Gobiesocidae (clingfish). The trap was to be used close inshore from rocks, wharves and boats. Hence it needed to be light yet robust enough to withstand the normal wear and tear of field handling. The light-traps referred to earlier that were designed for collecting aquatic insects etc., were too small to collect large numbers of fish larvae. A trap similar to that used by Davies (1954) and Baker (1972) for collecting pilchard larvae was available. However, this was too cumbersome for my purpose. Furthermore, the total collecting area of the cones was small relative to the area of the trap radiating light, thus decreasing the catching efficiency. I have used a sealed light on the end of a long pole, the light being placed just under the water to attract fish larvae that were then captured by a dipnet. This method, while effective in rocky shore surge channels not suitable for set traps, did not give any quantitative assessment, however rough, of larval number. Furthermore, light traps have an advantage in that they can be left unattended for several hours.

The trap needed to be submersible to different depths, portable and useable in the sea. Certain basic problems therefore had to be overcome. These were principally the sealing of the light, corrosion, release of water when retrieving the trap, loss of or damage to specimens, and weight of water causing stress on the apparatus. In addition to solving these problems, it was necessary to devise a system whereby the greatest cone aperture area (effective collecting area) was presented to the organisms. Perspex, because of its transparency and durability, was chosen as the main building material.

Description and Method of Construction

The trap (Figs. 4 and 5) is essentially a squat perspex ‘box’ with sides that form the outer aperture of four cones. The cones taper to narrow slit-like inner apertures that are in line with a light in the centre of the apparatus. All sides of the trap, therefore, act as effective collecting areas. The top of the ‘box’ is opaque to restrict radiating light to the areas of the cones; organisms are then not likely to approach the light from above the trap but rather from the side in line with the cones. The gauze ‘cod-end’ (bottom of the ‘box’) is four-sided and tapered, ending in a terylene collar designed to hold an Agee jar (Fig. 5). The corners are joined and strengthened by terylene tape. The upper edge of the net is bordered by a 3 cm wide terylene strip, folded from a 6 cm strip. The size of the mesh used for the ‘cod-end’ will depend on the size of the animals to be collected (300 microns for this trap), but mesh size must be large enough to allow reasonably rapid drainage of water from the light-trap. If not the weight of the water retained in the ‘cod-end’ will place undue stress on the whole apparatus. A mesh ‘cod-end’, as distinct from some solid equivalent, has the added advantage of being easily folded. The trap is thereby more readily handled.

The sealed light unit is fastened to the centre of the laminated top (Figs. 4 and 5, A₁ and A₂) and gives maximum radiation of light through the cones. The light (Fig. 1) consists of a brass holder, the lower portion of which is hollow and threaded to accommodate a small jam jar. This is sealed by pressure against a flat rubber ring. The flex enters the jar through the upper cylindrical stem of the brass holder, and effective sealing is obtained with an ‘O-ring’ forced tightly about the flex by a short bolt. Vaseline is spread liberally over the seals and the rim of the jar. Inside the jar there is a simple brass bulb holder that is screwed firmly to the brass unit, and receives two wires from the flex. Klinger (in Hale, 1953) used a 5,000 watt light, hoping to attract a great variety of marine life. Instead he found this very strong light less effective in attracting organisms than a simple hand-held flashlight. Foxon (1936) and Hale (1943) both found that cumaceans were attracted most effectively by low intensity light. They found that cumaceans tended to shun the brightest areas, preferring to remain on the periphery of the radiated light. A low intensity light (12v 6w) was therefore chosen for this trap.

The cylindrical projection of the brass light holder passes through the laminated top of the trap and is held securely in place by four screws. The light trap is suspended by four bridles, each 54 cm. long with spliced loops at each end. How the bridles are attached page 100

Fig. 1

page 101 to the trap is a matter of personal choice; in this case aluminium U-sections were used (Fig. 3, No. 1). The free ends of the bridles are gathered together and fastened to a galvanised shackle, which in turn is attached to the 10 m warp. The length of the warp will vary according to the height of the wharf or boat and the depth of the water to be sampled. The flex from the light unit is attached at intervals to the warp. The light trap is constructed in the following way:

Fig. 1

Operation

This light trap is light and will swing about with the slightest swell. Weights, perhaps those used by divers, may be added to each corner to increase stability. The time the trap is left submerged depends on the locality and type of animal sought. When collecting fish larvae it is advisable to restrict submersion time to no more than half an hour. Any longer and the small delicate larvae are rapidly consumed by isopods etc., and also damaged by general overcrowding. On retrieval the trap should be brought slowly to the surface so that the perspex skirt is just clear of the water. As the ‘cod-end’ is lifted from the water the whole unit is moved from side to side so as to keep the organisms swimming and thus prevent them becoming entangled in the mesh sides. This factor is more important with ‘many-legged’ crustaceans that become readily enmeshed. With the bottle clear of the water the unit may then be brought rapidly up to the wharf.

The light trap has also been successfully used to collect amphipods (Dr. A. A. Fincham, V.U.W.) and epitokous polychaetes (Mr. G. Read, Marine Laboratory, Island Bay, V.U.W.). In addition various adult teleosts, zoea and megalopa larvae and isopods have been found in differing numbers in samples taken. It is likely that the unit will collect any small positively phototropic aquatic organism.

Acknowledgements

I would like to thank Dr. P. H. J. Castle, Department of Zoology, Victoria University of Wellington, for his constructive criticism of this paper. I thank Mr. M. Loper, V.U.W., for the design and construction of the sealed light unit, and Mr. G. Grainger, Zoology Technician, Marine Laboratory, Victoria University of Wellington, for the construction of the light trap.