Benoit's Lantern Fish

Hygophum benoiti

This is a medium-size lanternfish known to reach a size of about 55 mm (Nafpaktitis et al., 1977); maximum size in the Ocean Acre collections is 44 mm. Hygophum benoiti is known only from the North Atlantic Ocean, where it is a temperate-semisubtropical species (Nafpaktitis et al., 1977) and, according to Backus et al. (1977), is among the dominant lanternfishes only off the west coast of Africa at about 10° to 25° N (Mauritanian Upwelling). It is one of the “very abundant” lanternfishes found in the study area, and was among the nine most abundant lanternfishes at each of the three seasons, ranking first in late summer (Table 131). The Ocean Acre collections contain 2850 specimens; 2522 were taken during the paired seasonal cruises, 1814 of these in discrete-depth samples, 1509 from noncrepuscular tows.

DEVELOPMENTAL STAGES.—Postlarvae were 6–12 mm, juveniles 9–23 mm, subadults 20–36 mm, and adults 24–44 mm. Sex determination was possible in almost all juveniles larger than 16 mm, but in very few smaller than 15 mm. External sexual dimorphism was first apparent in fish 19–22 mm; males larger than about 19 mm have a single large supracaudal luminous gland, and females larger than about 22 mm have 2–4 smaller infracaudal luminous glands. Adult females contained eggs as large as 0.5 mm, but most eggs were 0.2–0.3 mm in diameter.

There appears to be geographical variation in both the maximum size and size at maturity. Adults caught in the Mediterranean Sea were mostly 33–44 mm (Goodyear et al., 1972) but may grow as large as 48 mm (Taaning 1918); those in the Ocean Acre collections are mostly smaller than 40 mm, although a few are 44 mm; and most gravid females from the North Atlantic (no specific locality given) examined by Nafpaktitis et al. (1977) were 40–50 mm.

REPRODUCTIVE CYCLE AND SEASONAL ABUNDANCE.—Hygophum benoiti is an annual species with a marked peak of spawning in spring. Although the breeding season may extend from winter to summer, most spawning apparently occurs in April-May, when the bulk of the population is approaching one year of age. Abundance was greatest in late summer, intermediate in winter, and least in late spring (Table 66). The great abundance in late summer was due to juvenile recruits, which accounted for almost the entire catch, with postlarvae accounting for the remainder. In winter subadults and adults were predominant, together comprising more than 90 percent of the catch. The meager catch in late spring was almost exclusively juveniles and postlarvae. These abundance relationships also were evident in the size composition of the catch in each season. Most specimens were 11–13 mm in late summer, 23–30 mm in winter, and smaller than 15 mm in late spring.

Eleven postlarvae 6–9 mm were caught in February at 40 m during the morning crepuscular period, indicating that at least some spawning occurred in December and January. Spawning at that time most likely was at a minimum, as only one juvenile was taken between February and June. Winter adults, which were all taken in February (none in January), presumably represent those fishes spawned earliest during the previous season, subadults those from the peak in April-May, and juveniles those from the end of the peak in June or July. Winter adults probably would ripen and spawn within two months. Most subadults are calculated to be about 8–9 months old and would spawn during the next peak in April-May. Juveniles would not reproduce until June or July and perhaps, in small numbers, even into August.

By late spring the winter population had mostly spawned and died. The scant catch, except for a few 38 mm fish, consisted of recruits smaller than 15 mm. Abundance was at a minimum because recruits from the April-May peak were still too small to be adequately sampled by the nets. About 40 percent of the total postlarvae were caught in late spring (June), and an additional 45 percent in July.

The catch in late summer was made up of recruits (except for a few taken with the open Engel trawl); most were recently transformed juveniles 10–13 mm. Abundance, then at its maximum, increased tenfold from the late spring level (Table 66).

Otoliths were examined in an attempt to age fish larger than 25 mm. Those taken from winter specimens 29–41 mm all had 7 to 9 well-developed rings. These rings obviously represent a sub-annual growth pattern. Their number suggests that they may represent monthly increments of growth. Daily, fortnightly, and monthly patterns of ring deposition have been observed in temperate fishes (Pannella, 1971), and daily, weekly, fortnightly, and monthly rings have been found on the otoliths of tropical fishes (Brothers et al., 1976; Struhsaker and Uchiyama, 1976).

SEX RATIOS.—Males were much more numerous than females in winter, with a ratio of 1.5:1 (Table 67). Sex ratios for the other two seasons are not considered reliable estimates of the actual values, because only two specimens taken in late spring could be sexed, and little more than 15 percent of those examined from late summer (most smaller than 20 mm) could be sexed.

The numerical dominance of males over females in winter did not hold for each of the three older stages. Adult males were nearly 23 times more numerous than adult females, but female subadults were about 1.5 times more numerous than males of that stage. Both are significant differences from equality. These differences in part may be due to difficulties in staging males, but combining the two stages still resulted in a significant difference from equality, males being about 1.6 times more numerous than females. Juvenile males were more numerous than juvenile females but not significantly so. These differences do not appear to be a result of sampling bias. Males were often more numerous than females in discrete-depth samples made at night in the upper 50 m, where more than 90 percent of the catch was taken (Table 68). Males also were more numerous than females in most day samples.

The numerical dominance of juvenile females over juvenile males in late sumer is mostly due to 14–17 mm specimens, for which the female to male ratio is 3.1:1, a significant difference from equality. It is possible that females can be recognized at a smaller size than males.

VERTICAL DISTRIBUTION.—Daytime depth of occurrence in winter was 451–850 m with maximum abundance at 501–550 m, in late spring 551–1050 m with a slight concentration at 751–800 m, and in late summer 451–1100 m with a maximum at 701–850 m. Depth range at night in winter was 18–350 m with maximum abundance at 18–50 m; in late spring 50–250 m, 451–500 m, and 701–1050 m with a maximum at 751–800 m; and in late summer 51–1000 m with a maximum at 651–700 m (Table 68).

Stage stratification was evident in winter and late spring, with size stratification evident at all three seasons. By day in winter juveniles and subadults were most abundant at 501–550 m and adults at 701–750 m. Adults were not taken at as shallow a level as juveniles and subadults, and juveniles were not caught as deep as the older two stages. In late spring only postlarvae were taken above 700 m and only juveniles below 850 m.

In terms of size, in winter fish 30 mm and larger were not taken at either depth extreme; those 19–23 mm were taken only between 501 and 650 m; and fish 24–29 mm were taken over the entire vertical range. The smaller mean size of the catch at 501–550 m and 601–650 m, than at other 50 m intervals (Table 68), reflects growth in size in the population from mid-January to late February rather than size stratification. Fish caught in January had an average standard length (SL) of 23.7 mm and those in February (one year later) a mean of 27.7 mm; the difference presumably represented growth during a period of roughly 1.5 months.

The few specimens caught during daytime in late spring appeared to be stratified by size; those caught above 750 m were 9–10 mm and those caught at greater depths were 9–14 mm, including a 38 mm adult. Fish taken below 750 m (excluding the 38 mm adult) averaged 3.4 mm larger than the ones (9.6 mm) from shallower depths. In late summer during the day only fish smaller than 13 mm were caught at or near either depth extreme, where the catches were small. At intermediate depths the size range was 11–22 mm, and the mean size noticeably larger than that at either shallower or greater depths (14.8 vs 11.2 and 10.5 mm, respectively).

Stratification was not evident at night in winter, when about 93 percent of the catch was from 18–50 m; each stage and all sizes taken were most abundant at that depth. In late spring only postlarvae were caught at the shallow extreme and only juveniles at the deep extreme, but the catches were small at both depths. Size stratification was evident at night in late spring, with the catch from the upper 100 m consisting only of fish 6–9 mm, and that at 751–1000 m of specimens 10–14 mm. A 38 mm adult was taken at 201–250 m. In late summer the catch above daytime depths had a size range of 8–21 mm and an average SL of 14.0 mm, and at day depths the size range was 9–19 mm, and the mean size 11.7 mm. Only 7 of the 357 specimens caught at diurnal depths exceeded 13 mm in length. Larger fish from both depth strata were most abundant at or near the upper depth limit, and the mean size of the catch at 51–100 m and 451–700 m was larger than that at 101–400 m and 701–1000 m, respectively (Table 68).

Postlarvae probably do not undergo extensive diel vertical migrations. They were stratified according to size at night in late spring and late summer. Those caught in the upper 150 m were 6–11 mm and averaged 7.9 mm; those caught deeper than 500 m were 9–10 mm. At intermediate depths, postlarvae were 8–12 mm. Postlarvae were caught during the day only at 551–800 m in late spring. They were all 9–10 mm, as were night specimens from the same depths. The absence of postlarvae in day samples from the upper 150 m in late spring and at all depths in late summer almost certainly is due to the lack of samples at appropriate depths. In winter (February-March) the few postlarvae caught, all 6–9 mm, were taken at sunrise at 40 m, suggesting that smaller postlarvae probably are found in superficial waters both day and night in late spring and late summer. Postlarvae apparently spend the early stages of their life in the upper 150 m, and at 8–10 mm they descend to depths greater than 500 m where they transform into juveniles.

Diel migration was apparent at all three seasons; in late spring and late summer most of the population was nonmigratory, but in winter all except postlarvae were migrants. This is a consequence of the size and degree of development of the individual fish. Postlarvae and 9–10 mm juveniles are nonmigrants, and fish larger than 20 mm are regular diel migrants. At intermediate sizes, juveniles 11–13 mm are mostly nonmigrators, and those 14–19 mm are mostly migrators. Only 9 of 360 nonmigrants were larger than 13 mm. Migratory behavior first is assumed at a size of 11 mm and is almost universal at about 14 mm. This is well illustrated at night in late summer, when slightly more than 80 percent of the catch was from diurnal depths. About 85 percent of the 11–13 mm juveniles, but less than 20 percent of those 14–19 mm, were caught at these depths.

Little can be said concerning the chronology of diel vertical migrations at any season. Few H. benoiti were caught in the upper 250 m at night in late spring, and most of these were postlarvae, which probably do not migrate.

Although most of the winter population migrated vertically, the depth of maximum abundance (18–50 m) was not sampled until shortly before midnight, so arrival times at nocturnal depths must be estimated. Specimens were taken at day depths in a trawl made from 1.7 to 0.5 hours before sunset, and fish were caught at about 225 m and 125 m no later than 0.7 hours after sunset. Assuming that migrations were started at about 1.7 hours before sunset, roughly 2.5 hours were spent in transit between day depths up to 125 m. Allowing an additional 0.5 hour to reach the upper 50 m, a total upward migration of roughly 3.0 hours and a migration rate of about 165 m per hour between day (501–550 m) and night (18–50 m) depths of maximum abundance are obtained. The reverse migrations commence no earlier than about 1.7 hours before sunrise, and daytime depths are reached by no later than 1.8 hours after sunrise, resulting in a maximum migration time of approximately 3.5 hours. Using this estimate, a minimum rate of downward migration of about 145 m/hour is obtained.

The low proportion of migrants in late summer made it difficult to estimate migration times. A series of three one-hour evening crepuscular samples was made at 470–520 m; this is above the major concentration of H. benoiti during daytime (Table 68). The first sample, beginning about 2.2 hours before sunset, fished at 500–520 m and caught 46 specimens; the following two, both at 470–500 m, caught 38 and one specimen, respectively. This suggests that upward migrations commenced between 1.2 and 2.2 hours before sunset. Specimens were taken at about 325–350 m no more than 1.6 hours before sunset and at about 250 m no more than 1.2 hours before sunset, suggesting that the earlier estimate of starting time may be the more accurate one.

Sampling was inadequate to determine arrival times at the depth of maximum concentration of migrants (151–200 m). At 19 m a single specimen was taken no later than 0.8 hours after sunset, and an additional 5 specimens no more than one hour later. Assuming that upward migrations commence 2.5 hours before sunset and terminate 1.5 hours after sunset, they take about 4.0 hours to complete, occurring at a rate of about 150 m/hour between the day depth of maximum abundance (751–800 m) and the depth of maximum abundance of migrants at night (151–200 m). Downward migrations apparently start no more than about 2.5 hours before sunrise, as specimens taken at near the following depths between about 1.5 and 2.5 hours before sunrise would indicate: 45 m, 65 m, 125 m, and 275 m. Daytime depths were reached by 1.7 hours after sunrise, giving a downward migration time of about 4.0 hours, which is similar to the time for evening migrations.

PATCHINESS.—Patchiness was indicated by day in winter at 701–750 m and 801–850 m, and in late summer at 751–800 m. In winter adults were most abundant at 701–750 m; in late summer juveniles, the only stage caught, were at 751–800 m. The depth of maximum abundance in winter, 501–550 m, was sampled only once, and nothing can be stated concerning patchiness at that depth.

Patchiness was more extensive at night, being indicated at 18–50 m in winter, and at 51–100 m, 151–200 m, 651–700 m, 751–800 m, and 851–1000 m in late summer. These include the depths of maximum abundance of all stages taken in winter (18–50 m), and of nonmigrant (651–700 m) and migrant (151–200 m) juveniles in late summer. Nonmigrators appeared to have a patchy distribution below 600 m at night in late summer, a marked contrast to the daytime situation at that depth.

Significant CD values obtained for day samples at 751–800 m in late spring and for night samples at 51–100 m in winter and 301–350 m in late summer are thought to be due to factors other than patchiness. In late spring the four samples made at 751–800 m were equally divided between two cruises; neither set had a significant CD. Most of the specimens were taken during one of the two cruises, which suggests year-to-year variation in population density rather than patchiness. Winter night samples made at 51–100 m caught from 0 to 8 specimens, with only 2 of 11 samples having more than 3 specimens. These low numbers indicate that patchiness at this depth occurs only on a small scale. Three of the four samples made at 301–350 m at night in winter were taken during one of the two cruises and accounted for only one of the 25 specimens caught at that depth, suggesting that year-to-year variation in population density rather than patchy distribution was responsible for the large CD value.

NIGHT:DAY CATCH RATIOS.—Night-to-day catch ratios for discrete-depth captures, including interpolated values, were 0.9:1 in winter, 2.0:1 in late spring, and 3.8:1 in late summer (Table 69). Except for adults, each stage taken followed the overall seasonal trends.

The winter ratio is somewhat deceptive, as the night to day catch ratios for each stage were more deviant from 1:1 than the ratio for total specimens (Table 69). Juveniles and subadults were more abundant in day than in night samples; adults the reverse.

Except for adults in winter, the ratios do not seem to be related to diel changes in vertical range. The depth range of adults at night in winter was compressed compared to that of the day, with a large concentration within the 18–50 m stratum. In all other cases abundance was greater in samples made during the diel period in which the depth range was more extensive.

Net avoidance does not seem to be an important factor in the differences between day and night catches. Most fish caught in late spring and late summer were smaller than 20 mm and are not likely to be able to avoid the nets to any great degree. Larger fish formed a good portion of the catch only in winter, but specimens 30 mm and larger, which should have greater ability to avoid the nets, were about equally abundant in day and night samples in winter. The greatest diel difference in abundance in winter was obtained for fish 23–24 mm.

Diel differences in clumping may have contributed to the discrepancy between night and day catches in late summer. Clumping was indicated to be much more extensive at night than by day in late summer, but the difference between day and night catches was so large that it is almost certain that other factors were involved. The catch at each 50-m interval between 650–1050 m sampled both day and night was greater at night, suggesting that the daytime depth of maximum abundance was not sampled. Enhanced daytime avoidance probably was not responsible for this difference. At 650–1050 m the catch during daytime ranged in size from 10 to 22 mm and averaged 14.4 mm, and at night the size range was 9–18 mm with a mean size of 11.8 mm.

The ratio for late spring is based upon a total of 36 specimens, all but 4 of which were caught during one of the two cruises.