Tuatara: Volume 21, Issue 3, April 1975
A LIGHT-TRAP FOR COLLECTING MARINE FISH LARVAE AND INVERTEBRATES
A LIGHT-TRAP FOR COLLECTING MARINE FISH LARVAE AND INVERTEBRATES
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.
|(1)||Cut the collecting cones, top and sides (Figs 2 and 5, A-E), from a single sheet of 5 mm thick clear perspex measuring at least 116 cm X 91 cm (Fig. 2). When cutting, allow for the width of the saw cuts. Smooth or bevel the rough edges using medium (not fine) sandpaper.|
|(2)||Form the top of the trap by laminating A1 and A2 (Fig. 5). As an alternative the top may be cut from a single sheet of 1 cm. thick black perspex. If transparent perspex is used, then paint the top black.|
|(3)||Glue the sides of the cones C1-4 at right angles to the top and bisecting each corner. The edges to be glued should be bevelled to provide a larger glueing surface (Fig. 3, No. 2). Triangular wedges of perspex are added to increase the strength of the joints.|
|(4)||The upper parts of the cones B1-4 are now glued to the laminated top and to the cone sides C1-4.|
|(5)||Glue the lower parts of the cones B5-8 to the angled edges of the cone sides C1-4. The size of the internal cone aperture may be altered by increasing or decreasing the angle of the upper and lower cone portions (B1-4 and B5-8).|
|(6)||Assemble the perspex skirt by glueing the series D1-4 to each other to form a square. D3 and D4, being 1 cm shorter, lie inside D1 and D2.|
|(7)||The skirt is now glued to the bottom edges of B5-8 (lower edge of the cones), being further held in place by triangular perspex fillets (F) cut to fit on the inside between the skirt and the ventral plates of the cones.|
|(8)||Drill 16 5 mm diameter holes through the top 5.5 cm apart and lying in from a line representing the innermost border of the cones. These produce a downcurrent of water that helps prevent organisms from passing out of the cones as the trap is lifted through the water column. Further holes of similar diameter are drilled in each corner of the top to allow air to escape from the trap as it is lowered into the water.|
|(9)||Drill 20 5 mm diameter holes in the perspex skirt D1-4 and the backing plates E1-4 which correspond to a similar series of holes in the upper border of the gauze ‘cod-end’. The ‘cod-end’ may be constructed according to personal preferences and therefore requires no further explanation here.|
|(10)||Slide the terylene border of the ‘cod-end’ over the skirt and hold in place with the back plates E1-4 and the 5 mm X 2 cm galvanised bolts.|
|(11)||Fasten the sealed light unit to the laminated top of the light trap.|
|(12)||Bolt the bridle attachments to the corners of the top of the trap, at the same time fastening the bridle warps and flex.|
|Clear perspex||5 mm X 91 X 116 cm (NZ 3 X 4in. stock)|
|1 galvanised shackle|
|1 12 v. battery|
|Nylon bolting cloth||40 X 72 cm (300 μ mesh)|
|Terylene tape||140 X 3 cm|
|Terylene cloth — collar||35 X 11 cm|
|— border||184 X 6 cm|
|Galvanised nuts and bolts||5 mm X 2 cm (X 20)|
|Self-tapping galvanised screws||5 mm X 3 cm (X 4)|
|Propylene rope — bridles||300 cm|
|— warp||10 m|
|Aluminium U-section — brackets||1.5 X 2 X 16 cm|
|1 sealed light unit with 12 v. 6 w. bulb|
|Insulated flex||10 m|
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.page 104
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.
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.
Baker, A. N., 1972: Reproduction, early life history, and age-growth relationships of the New Zealand pilchard, Sardinops neopilchardus (Steindachner). Fish. Res. Bull., (5): 64 pp., 39 fig., 9 tab.
Davies, D. H., 1954: The South African pilchard (Sardinops ocellata). Development, occurrence and distribution of eggs and larvae, 1950-51. Investl. Rep. Div. Fish. Un. S. Afr. 15: 28 pp.
Espinosa, L. R., and Clarke, W. E., 1972: A polypropylene light trap for aquatic invertebrates. Calif. Fish Game, 58 (2): 149-152, 2 fig.
Foxon, G. E. H., 1936: Notes on the natural history of certain sand-dwelling Cumacea. Ann. Mag. nat. Hist., 10 (17): 377-393, 7 fig.
Hale, H. M., 1943: Notes on two sand-dwelling Cumacea (Gephyrocuma and Picrocuma). Rec. S. Aust. Mus., 7 (4): 337-342, 9 fig.
—, 1953: Notes on distribution and night collecting with artificial light. Trans. R. Soc. S. Aust., 76: 70-76, 3 fig.
Hungerford, H. B., Spangler, P. J., and Walker, N. A., 1955: Subaquatic light traps for insects and other animal organisms. Trans. Kans. Acad. Sci., 58 (3): 387-407, 2 fig.
Parsons, L. S., and Hodder, V. M., 1970: Occurrence of juvenile and spawning Atlantic Mackerel in Southeastern Newfoundland coastal waters. J. Fish. Res. Bd. Can., 27 (11): 2097-2100.
Washino, R. K., and Hokama, Y., 1968: Quantitative sampling of aquatic insects in a shallow-water habitat. Ann. Ent. Soc. Amer., 61 (3): 785-786, 1 tab.
Winn, H. E., and Miller, R. R., 1954: Native postlarval fishes of the lower Colorado River Basin, with a key to their identification. Calif. Fish Game, 40 (3): 273-285, 1 fig., 1 tab., 4 pl.