Bigscale

Body slender blunted anteriorly, eye small. Gas bladder longer than stomach (Ref. 37108).

Dorsal spines (total): 2 - 3; Dorsal soft rays (total): 112; Analspines: 1; Analsoft rays: 7 - 8

Trawled below 500 m (Ref. 4241).

Trawled below 500 m.

fisheries: of no interest

Scopeloberyx opisthopterus

A fairly small species, S. opisthopterus attains a maximum length of about 39 mm. It occurs in the Atlantic from about 10°S to 40°N, but appears to be absent in the polygon between the equator and 30°N bounded by 30° and 60°W. This was the second most abundant melamphaid in the Ocean Acre area at all seasons; only M. pumilus was more abundant. A total of 595 specimens was taken, 206 of them in discrete-depth samples, 195 from noncrepuscular samples during the paired cruises (Table 147).

DEVELOPMENTAL STAGES.—The 595 specimens include 19 postlarvae, 241 juveniles, 84 subadults, and 251 adults. Postlarvae possess small melanophores scattered over their otherwise unpigmented bodies and range from 4–13 mm. Juveniles are 12–27 mm, subadults 25–32 mm, and adults 29–39 mm.

Sexual dimorphism in size exists in this species, females being slightly larger than males. Subadult males are 25 to 31 mm (mean 27.6 mm), females 27 to 32 mm (mean 29.9 mm). Adult males are 29 to 37 mm (mean 31.7 mm), females 29 to 39 mm (mean 34.7 mm).

SEX RATIOS.—Among juveniles, males outnumbered females at every season represented by a reasonable sample, although the difference was statistically significant only for June (Table 164). The same tendency characterizes subadults, but is less obvious because of the small number of specimens of this stage. Among adults, the tendency appears to reverse, with females more abundant than males during the first six months of the year. This suggests a greater male mortality prior to or just after spawning. From July through December, an equalization or reversal again appears, possibly due to the gradual decimation of postspawning females.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Scopeloberyx opisthopterus appears to breed at least from spring through summer, as reflected by the occurrence of postlarvae from June to September (Table 163). The life cycle may be deduced from the seasonal length-frequencies (Table 165). The winter length-frequency is bimodal, one mode (30–39 mm) comprising adults that presumably will breed in the spring and summer, the other mode being medium-size juveniles (16–22 mm) recruited from the past year's spawning. In late spring, night data especially show one mode (22–28 mm) including both large juveniles and subadults that probably represent the medium-size winter juveniles, and a second mode in the adult size range (32–34 mm), which may represent the later-spawning adults. The paucity of large adults (35–39 mm) in late spring suggests that these have died after an earlier spawning. The data for late summer show three modes: one comprised mostly of large juveniles (23–25 mm), presumably representing the smaller juveniles of late spring; one of subadults and small adults (29–33 mm), presumed to represent the larger juveniles from late spring; and one of postlarvae and very small juveniles (12–15 mm) that will become the larger winter juveniles.

A 2-year life cycle is suggested by the progression of size modes. Postlarvae (8–9 mm) just entering the catch in late spring form a mode at the smallest juvenile sizes (12–15 mm) in late summer, which progresses to the mid-juvenile stage (16–22 mm) in winter, to the large juvenile-subadult stage (22–28 mm) by the following spring. In late summer of their second year this modal group attains the small-adult stage (29–33 mm) that will become the large adults of winter and spawn in the spring of their second year.


The possibility of two spawning peaks is suggested by similar progression from the small juveniles (14–17 mm) in late spring, to the large juveniles (12–27 mm) of late summer and the small adults (29–33 mm) of winter that will be the late spring and summer spawners of the coming season.

Scopeloberyx opisthopterus was most abundant in winter, least abundant in late spring, and slightly more abundant in late summer than in late spring (Table 166). These estimates of relative abundance probably are reasonable for most juveniles, subadults, and adults, but postlarvae and the smallest juveniles are not adequately sampled. The figure for winter probably represents the actual population, with juveniles and adults representing the 2-year classes, and with postlarvae and subadults absent or scarce. In late spring, much of the adult population is assumed to have spawned and died, accounting for the considerable decline. By late summer, the subadult and adult populations are further decimated, but juveniles, representing the offspring of the year's spawning, have begun to enter the catch and counterbalance the loss of adults. If postlarvae had been sampled adequately, their greatest abundance probably would have been in late summer, with late spring intermediate, and winter least.

VERTICAL DISTRIBUTION.—Day and night vertical distributions were essentially similar during all three seasons (Table 167), indicating that this species does not migrate vertically. Smaller postlarvae occur between 50 and 300 m. Larger postlarvae and all other stages inhabit depths between 800 m and at least 1550 m. Specimens were caught in open nets fishing as deep as 3500 m, but these could have been taken during the oblique portions of these tows. Two adult males and one adult female were caught at night in the late spring at 201–250 m. It is not likely that these specimens were contaminants, for the preceding five tows were made at 200–400 m. Presumably, these individuals were strays.

Stage stratification is suggested, with juveniles occurring mainly from 850–1050 m and subadults and adults concentrating in deeper water (Table 167). The latter two stages were taken over the full depth range of the species, occupying a greater range than juveniles, but tended to be most abundant below 1100 m.

PATCHINESS.—Coefficients of dispersion are not significant for any 50-m depth interval at any season or time of day, which can be interpreted to indicate lack of clumping. However, the exceptionally large catch of juveniles in two winter night samples from the same trawl and depth, and the absence of juveniles in winter daytime catches, suggest that clumping may, indeed, characterize juveniles.

NIGHT:DAY CATCH RATIOS.—The ratios of night to day discrete-depth catch rates, using interpolation in unsampled depth intervals, were 4.1:1 in winter, 1.1:1 in late spring, and 0.5:1 in late summer (Table 168). In late spring, when the ratio approached 1:1, juveniles, subadults, and adults were in comparable numbers and sizes both day and night. In both late summer and winter, entire size ranges were absent in samples made during the diel period with the lower catch rate but present in the other period. In late summer the gaps occurred at night at 16–22 mm (medium-size juveniles) and at 27 mm and larger (subadults and adults). In winter a gap occurred during daytime at 16–25 mm (all juveniles). The results are not supportive of enhanced day or night net avoidance, but point to sampling deficiencies as the probable cause.

Atlantic: in tropical waters, as far north as Grand Bank

Trawled below 500 m.

nektonic