Age and Growth of the Nehu, Stolephorus Purpureus (pisces: Engraulidae), from the Hawaiian Islands as Indicated by Daily Growth Increments of Sagittae

Direct evidence is presented that the sagittae of nehu, Stolephorus purpureus, grow by discernible daily incrementa. Agmg by daily growth incrementa provides the means to establish a general growth curve for the filat 6 mo of life for this species. Adult nehu exhibit nearly linear growth between 30 and 60 mm standard length. Preliminary evidence is presented that the nehu population of Pearl Harbor may grow more rapidly than that of Kaneohe Bay. Attempts to age tropical fishes by conventional methods have generally been thwarted by the absence of well-defined annuli in calcarious structures and protracted spawning periods which make length-frequency mode progression analyses difficult. Recognizing that exceptions to the above statement exist, Pannella’s work (1971) providing indirect evidence of the presence of daily growth layers and periodical deposition patterns in the sagittae (otoliths) of three species of boreal fishes from the western North Atlantic suggested a means for conducting age and growth studies of tropical species. He concluded in that report: “Preliminary observation of growth patterns in sagittae of other species, living at various depths and different climates, appears to support the idea that daily growth may be a universal feature of fish otoliths.” Pannella’s (1974) later work in Puerto Rico provided circumstantial evidence of daily growth layers in sagittae of several species of tropical fishes. To gain direct evidence that daily growth increments exist in tropical fishes we studied the nehu, Stolephorus purpureus Fowler, a small engraulid endemic to the Hawaiian Islands. The nehu is the basis of a live-bait fishery producing about 4,000 metric tons annually of skipjack tuna, Kutsuwonus pelurnis (Linnaeus), from the vicinity of the Hawaiian Islands. Stolephorus purpureus is a short-lived species (less than 1 yr) and has been the subject of relatively numerous studies: Nakamura (1970) has summarized the biological ‘Southwest Fisheries Center, Honolulu Laboratory, National Marine Fisheries Service, NOM, Honolulu, HI 96812. knowledge of this species available through 1965. Our work provides evidence of the presence of daily growth increments in the sagittae of nehu and permits the assembly of a growth curve for the first 6 mo of life for this species. Brothers et al. (1976) have recently demonstrated the presence of daily growth increments in larval Engraulis morahx Girard and Leuresthes tenuis (Ayres) and presented evidence that the phenomenon occurs in several other species of California fishes. METHODS AND MATERIALS The nehu samples were taken with three types of gear in Pearl Harbor and the southeastern end of Kaneohe Bay, Oahu, Hawaiian Islands. Adults and juveniles (> about 30 mm standard length (SL) ) were sampled with commercial bait seines (square mesh measuring 3.2 mm to a bar) in Pearl Harbor. Postlarvae (about 3 20 mm SL), juveniles, and adults were obtained in Kaneohe Bay by a similar seine having a bar mesh measurement of 1.6 mm. Larvae (< 20 mm SL) were obtained near Coconut Island by personnel of the Hawaii Institute of Marine Biology with 0.5-m ring nets with mesh sizes of 550 pm. Three separate holding experiments were conducted to test the hypothesis that the sagittae of nehu grow by discernible daily increments. All animals for these experiments were collected in Pearl Harbor and held in tanks of 38-kl capacity at the National Marine Fisheries Service (NMF’S) Kewalo Basin Facility. The tanks were supplied with well seawater of 23”-24”C and 33-35% salinity a t a rate of about 300 literdmin. The nehu 9 Manuscript accepted August 1975. FISHERY BULLETIN: VOL. 74, NO. 1, 1976. FISHERY BULLETIN VOL. 74, NO. 1 first holding experiment and those collected from Kaneohe Bay and Pearl Harbor during spring 1972 were placed in glycerine on slides and covered. Some erosion of the sagittae edges was noted after about 5 mo, and this practice was discontinued after the first experiment. Slides were either labeled with date of collection and length of fish or assigned a five digit random number for identification. Our initial counts were taken from thin sections of sagittae taken on the frontal plane. After mounting the sagittae in epoxy resin, the initial plane of polishing was made with rough sandpaper. As the surface approached the desired section, fine wet silicon carbide sandpaper (400 grit) was used. Final polishing of the surface was done with suspensions of aluminum oxide particles having diameters of 15, 5, and 0.3 pm. The section was thinned on the opposite side to a practical thickness and etched in a 1% solution of HCl for variable periods up to 3 min. A few attempts to make acetate peels of the small nehu sagittae sections as described by Pannella (1971) and Pannella and MacClintock (1968) were unsuccessful. We eventually abandoned the sectioning of sagittae because of the time required and the difficulty in obtaining a precise section from the nucleus to the posterior edge of the sagitta. Sagittae were obtained from larvae less than about 20 mm SL by placing the specimen on a slide and gently teasing the otoliths from the head region. The sagittae were then mounted in Euparal and read immediately. These otoliths tended to clear completely within a few hours, and photographs are the only permanent record of these specimens. The smallest growth increments of the mounted sagittae were counted with a compound microscope at magnifications of 400-800~. The smallest growth increment in all fish otoliths consists of both an organic and an inorganic layer (Degens et al. 1969). These two layers in the nehu otolith together measure about 1-4 pm thick. A zoom feature of the microscope was found to be extremely useful. Counts were maintained on a hand tally. Enumeration of the smallest growth increment layers in whole sagittae is tedious, and reliable counts can be obtained only after a moderate amount of experience has been acquired. Enumeration is, obviously, much easier in sagittae from smaller fishes (Figure 1). Usually, readings cannot be made in a direct line from the nucleus to the selected point on the edge of the sagitta; were fed with frozen and live brine shrimp, Artemia sp., under variable regimes as described below. Each experimental population of nehu was sampled during placement in holding tanks, and then subsampled at various time intervals as described for each experiment. Otoliths were extracted from most specimens within a few hours of sampling. The remaining samples were frozen in seawater or preserved in 75% solution of isopropanol until extraction of otoliths (removal of tissue from otoliths of alcohol preserved specimens is difficult). The first holding experiment was begun 5 April 1972. A 16-day sample (21 April) and a 34-day sample (9 May) were obtained from this population. The animals were fed once a day with frozen and/or live brine shrimp. The second holding experiment was begun 15 December 1972. This population was initially fed once a day. A high mortality was observed during the first 2 wk, after which food was provided twice daily. Samples were collected weekly after 1 mo of captivity. We examined sagittae from animals collected on 19 January and 26 January 1973. The third holding experiment was begun 4 May 1973. This population was fed two or three times daily with frozen brine shrimp. Samples were obtained weekly between 4 May and 6 July. We examined sagittae from animals collected 25 May and 8 June 1973. Wild populations of larval, juvenile, and adult nehu were sampled 13 times in Kaneohe Bay between 19 March 1972 and 13 July 1973 to obtain estimates of growth rates at various seasons. Although a second species of Stolephorus (S. buccaneeri Strasburg) occurs in Hawaii, larvae of this species have not yet been collected in the southeastern end of Kaneohe Bay (Watson and Leis 1974; W. Watson pers. commun.). After extraction, the sagittae were cleaned and etched for up to 3 min in a 1% solution ofHC1, then washed and mounted whole on glass slides with the mounting medium EuparaP and covered with glass cover slips. Short lengths of monofilament line were used to prevent the contact of the specimen by the cover slide. Although the smallest growth increments are microscopically discernible immediately after extraction their detection was enhanced after about 30 days of clearing in the mounting medium. Sagittae used in the zReference to trade names does not imply endorsement by the National Marine Fisheries Service, N O M .