Collection of the hagfish

The hagfish, Eptatretus burgeri Girard, were trapped two to eight times a month at depths of 6 to 10 m in Moroiso Bay (Fig. 1, A) and Koajiro Bay (Fig. 1, B) near MMBS (35°09′18″ N, 139°36′54″ E), and at depths of 50 m (Fig. 1, C) and 100 m (Fig. 1, D) in Sagami Bay (Fig. 1, inset). Depths of 50 and 100 m are found at about 1,600 m and 4,800 m west to the MMBS, respectively. Collections were performed during October1970 to March 1973. Months of collections for each experiments are listed in Table 1 and are described in the sections of respective experiments. Collection in Moroiso Bay was not carried out after December 1971, because of small size of the population of the hagfish there (Table 2).

Table 1

Experimental periods in respective experiments

Table 2

Dates of collection and number of hagfish caught in Moroiso Bay and Koajiro Bay

For capturing the hagfish, a cylindrical trap (80 cm in length, 15 cm in diameter) with a funnel-shaped one way entry at both ends was used, baited with sardines. Before experiments, we confirmed twice that the hagfish could not be caught in daytime in shallow water, although we knew that they are nocturnal (Dean, 1904; Kobayashi et al., 1972; Ooka-Souda et al., 1985). Therefore, 6 to 39 traps in shallow water and 2 to 4 in deeper water, respectively, were placed at the sea bottom, separated by 10 to 100 m distances from each other, depending on the depth of the sea, in late afternoon, and picked up in the following morning. Incidentally, although hagfish could be caught during daytime in deeper water of 50 or 100 m, traps were placed in the late afternoon as well as in shallow water. Sardines were the best bait compared with mackerel, cod oil soaked in cotton and viscera of hagfish.

Marking experiments for migration studies

Two hundred and nine hagfish more than 40 cm in total length, which were expected to migrate to shallow water (see Fig. 5), were trapped in water of 50 m depth (Fig. 1, C) in October and November 1971. One thousand three hundred and twenty three hagfish of all total lengths were caught in Koajiro Bay (Fig. 1, B) in April, May and June 1972. For the marking, a small triangular piece was cut off from the upper or lower margin of the tail fin. Immediately after the marking, the hagfish were released at the same place where they were caught. Months during which recapture of the marked hagfish occurred are described in Results. The mark remained visible for at least about 5 months.

Measurement of total length and observation of gonads

To examine the relationship between migration, body length and sex, total lengths of all hagfish caught were measured. All hagfish, except for those used in the marking experiments, were dissected and sex was examined.

Sweeping of the sea bottom by net

In an attempt to locate the spawning ground and to obtain fertilized eggs or juveniles, net sweeping was carried out in October 1973 at depths of 80 to 110 m and in November 1975 at depths of 40 to 80 m (Fig. 1). The sweeping device was a set of 7 to 10 layers of nylon nets, each of which was about 20 cm in width, 4.5 m in length and 1 to 3 cm in mesh size. It was weighed at the bottom by a lead-weighed steel tube (2 m in width and 8 cm in diameter) at one end. The device was drawn very slowly by a boat to sweep the bottom surface for about 200 m distance in each trial. Such a device is used to collect corals and other benthic animals at the Station.

Examination of habitat

Substrate samples were collected from the bottoms of Koajiro Bay (water depth of 9 m) and of Sagami Bay (50 m) with a SK type bottom sampler (Rigosha, Tokyo). The sizes of the particles therein were measured with a sieve (Marukawa Co. Ltd., Tokyo), whose pore sizes were 3.36, 1.00, 0.50, 0.21 and 0.053 mm. The contents of the intestine of the hagfish caught in 50 m in Sagami Bay were collected and the sizes of the particle therein were also measured.

Statistics

Kolmogorov - Smilnov two-sample test was used to compare the frequency distribution histograms of the body length (1) of the hagfish caught in each month in Koajiro Bay, (2) of those caught in each month in water of 50 m depth, and (3) of the hagfish caught in Koajiro Bay and in water of 50 m depth. Student's t-test was used to compare the body length between males and females.

1. Collection in each month

In the shallow water of 6 to 10 m depths in Moroiso Bay and Koajiro Bay, the hagfish could be caught during a period from mid October to mid July of the following year, but not during any other period (Table 2). On the other hand, the hag-fish could be caught throughout the year in water of 50 m depth (see Fig. 4). Number of hagfish collected per trap was relatively consistent in respective collection sites: it was 8.1± 1.2 (± SEM, n=21 collections) in Koajiro Bay during November 1971 through May 1972, whereas it was 18.7± 1.9 (± SEM, n=37 collections) in water of 50 m depth during November 1971 through March 1973. Mean numbers of the traps used in one collection were 10.0± 1.0 (± SEM, n=47) in Koajiro Bay, and 3.3± 0.2 (± SEM, n=37) in water of 50 m depth during November 1971 through March 1973.

2. Marking experiments

In the first experiment, 209 hagfish caught in water of 50 m depth in October and November 1971, were marked. Among 764 hagfish caught during November 1971 to February 1972 in Koajiro Bay and Moroiso Bay, 65 marked hagfish were recaptured starting from November 1971. Recovery was 31.1%. In the second experiment, 1,323 hagfish caught in Koajiro Bay were marked in April, May and June 1972. Among 939 hagfish caught from May to September 1972, in water of 50 m depth, 22 marked hagfish were recaptured starting from May 1972. Among 74 hagfish caught during the same period in water of 100 m depth, one hagfish with the mark was recaptured. Recovery in water of 50 and 100 m depths was 1.7%. These observations indicate that the hagfish exhibit seasonal migration between shallow water less than 10 m in depth and water of 50 to 100 m in depth.

3. Sex difference in total length and sex ratio

There was no significant difference in the mean total length between males and females in each month either in Koajiro Bay or in water of 50 m depth, respectively. However, in total, females (37.6± 0.19 cm, n=1,564) were significantly larger than males (36.4± 0.24 cm, n=1,221)(p<0.05).

Based on 2,785 specimens caught in Koajiro Bay and water of 50 m depth during a period from November 1971 to March 1973, the ratio of males to females was 1:1.28; thus trapped females were significantly more numerous than males (p<0.001). However, in Koajiro Bay, almost all hagfish caught in October and most hagfish caught in November were females (Fig. 2). This indicates that females migrate first to Koajiro Bay during this period. From December to June, there was no difference in sex ratio except March in either Koajiro Bay or water of 50 m depth. The reason why number of trapped females is significantly larger than males in March is not explicable at the present time. Significant differences in sex ratio were also noted in November and September in water of 50 m depth, but such differences are not explainable at the present time. The patterns of the seasonal changes in the sex ratio observed in the present study are similar to those of Dean (see Conel, 1931).

Fig. 2

Percent of females of all hagfish caught in Koajiro Bay (closed circles) and water of 50 m depth (open circles) in each month. The percentage was calculated from the hagfish caught in each month from November 1971 to March 1973. Numbers of animals in each month were listed in Figures 3 and 4, although the sex was not shown. *, P < 0.05.

Fig. 3

Frequency distribution histograms of the total lengths of the hagfish caught in Koajiro Bay. Fishes caught each month from November 1971 to March 1973 were collected. Total lengths are expressed at 3 cm intervals. Numbers in each column indicate number of animals collected in the same months of the first and second years in order.

Fig. 4

Frequency distribution histograms of the total lengths of the hagfish caught in water of 50 m depth. Fishes caught each month from November 1971 to March 1973 were collected. Total lengths are expressed at 3 cm intervals. Numbers in each column indicate number of animals collected in the same months of the first and second years in order.

Fig. 5

Frequency distribution histograms of the total lengths of the hagfish caught during months from November to April in Koajiro Bay (upper panel) and in water of 50 m depth (lower panel).

4. Relationship between total length and migration

The largest trapped hagfish was 76.5 cm (male) in total length, which was caught in Koajiro Bay in April 1973. The smallest specimen was 9.3 cm (sex is not clear) caught in water of 100 m depth in September 1973.

5. Failure to find the spawning ground or to collect fertilized eggs

Sixteen net sweepings in total, which were carried out at depths from 40 to 110 m in October and November (Fig. 1), failed to collect eggs or juveniles. As shown partly in Figure 1, the depth of water increases gradually toward 100 m, and then increases suddenly to about 700 m. Therefore, the spawning ground may be in water of more than 100 m depth, and juvenile hagfish may spend their life there until they grow to about 20 cm in total length.

6. Characteristics of the habitat

In Koajiro Bay, the surface of the bottom is covered with mud to a certain depth. The mean particle size in the mud layer is less than 53 μm. Under this mud layer, there seems to be a layer of sand with mean particle sizes of 53–210 μm. In water of 50 m depth, the bottom surface seems to be covered by a layer of sand of less than 53 μm particles mixed with sediments to certain depth, and beneath it there seems to be a layer of sand with 53–210 μm particles. The upper sand layer contains protozoa and dead plankton. In the intestines of the hagfish caught in water of 50 m depth, fine sands of less than 53 μm particle size, protozoa, plankton and others were found. Therefore, it is possible that these microorganisms could be utilized as food in the hagfish living in deep water. Shelton (1978) also reported that the hagfish, Myxine glutinosa, are carnivorous, carrion feeders, and/or scavengers.

Migration general

Dean (1904) found that the hagfish, E. burgeri, inhabit shallow water of 6 to 10 m depths of Koajiro Bay except during September and October. However, we could not find them during July, August in addition to September and October in Koajiro Bay (Kobayashi et al., 1972; the present study). This difference which had occurred during 70 years is difficult to explain. Furthermore, we found that they live in water of 50 to 100 m depths in September and October and suggested that they engage in migratory behavior (Kobayashi et al., 1972). These findings were extended considerably by the subsequent studies. Namely, Fernholm (1974), by SCUBA diving, observed many hagfish in Koajiro Bay during the period from late October through early June, but no hagfish were found there during the remaining period. Patzner (1977, 1978) reported annual changes in the testis and ovary using the hagfish collected in Koajiro Bay from October to June and in water of 50 or 100 m depth from July to September. In another study, Tsuneki et al. (1983) also observed the seasonal migration and seasonal gonadal changes of this species near the Oki Islands in the Japan Sea. Thus, E. burgeri seems to migrate generally between shallow water and water of at least 50 to 100 m depths. However, all these reports mentioned above were inferred from indirect evidence that the hagfish were absent from shallow water during months from July to September, and that they were collected in water of about 50 m depth. In the present study, we obtained direct evidence from recovery of marked hagfish at sites different from the place of marking. This is the first report which indicates directly that there is the seasonal migration of the hagfish, E. burgeri, between shallow water of Koajiro Bay and/or Moroiso Bay and deep water more than 50 m of Sagami Bay. A high percentage of the recovery (31.1%) recorded in the hagfish which were marked in water of 50 m depth and recaptured in Koajiro Bay or Moroiso Bay, suggests that most hagfish living in Koajiro Bay and Moroiso Bay come from water of 50 m depth. On the other hand, only 1.7% of recovery was recorded in the hag-fish which were marked in Koajiro Bay and recaptured in water of 50 m or 100 m depth. This low percentage of recovery is considered to be due to the following complex of factors: 1) The number of the hagfish recaptured in water of 50 and 100 m depths was smaller (939 and 74, respectively) than those released (1,323), 2) the areas of water of 50 and 100 m depths were too wide to survey thoroughly, 3) collected hag-fish included those stayed there throughout the year, and 4) it may also be likely that marked hagfish moved to water deeper than 100 m depth. Despite such a low percentage of the recovery, the present result clearly shows that migration of the hagfish from Koajiro Bay to water of more than 50 m depth occurs.

In the present study, number of hagfish collected per trap was more than two times greater in water of 50 m depth than in Koajiro Bay. However, the distances between traps were less than 10 m in Koajiro Bay, whereas they were about 50 m in water of 50 m depth. Thus, it is not clear whether the difference of number of animals collected per trap between two collection sites reflect the difference of the population density between two or not.

Migration, body length and gonads

From collection data and marking data, it is suggested that (1) larger hagfish (≥39 cm) migrate from water of 50 m depth to shallow water of Koajiro Bay and Moroiso Bay after mid October, whereas smaller hagfish (< 39 cm) begin to migrate to shallow water after November, with peaks after March, and 2) larger hagfish (≥39 cm) begin to migrate to deep water from May, whereas smaller hagfish (< 39 cm) begin to migrate to deep water in June, and thus all hagfish complete the migration to deep water by mid July. In a separate study of this hagfish population (Nozaki et al., 2000), most hagfish larger than about 40 cm in length living either shallow or deep water have maturing gonads, and most females larger than 40 cm, which migrated to Koajiro Bay during October and November, have post-ovulatory follicular capsules. Thus, it is possible, therefore, that the migration is initiated by reproductive populations of the hagfish larger than 40 cm in total length in both ways.

The present study also showed that in water of 50 m depth the percentage of larger hagfish (≥36 cm) increased during the months from November to January. However, such increases in larger hagfish in water of 50 m depth are difficulat to understand, since they should be decreasing in numbers due to the migration to shallow water. One possible explanation is that they came from other places of origin such as water of 100 or more than 100 m in depth. It is further likely that some specimens were trapped on their way to shallower water, since one hagfish marked in Koajiro Bay was recaptured in water of 100 m depth.

Spawning ground

In a previous study (Kobayashi et al., 1972), we could not find hagfish in October at Koajiro Bay or Moroiso Bay, but found hagfish in water of 50 m depth. We considered that the spawning ground may be located in water of 50 m depth (Kobayashi et al., 1972). However, in the present study, net sweeping of the bottom of water of 50 to 110 m depths failed to collect eggs or juveniles. Further, hagfish less than 20 cm in total length were rarely found at water of 50 m depth. Therefore, the spawning ground may not be located around 50 m depth. It seems possible that it is located at water deeper than 100 m.

Factors initiating migration

Since smaller hagfish (<36 cm) with immature gonads (Nozaki et al., 2000) also migrate to shallow water of less than 10 m, the adaptive significance of the seasonal migration between shallow and deep waters does not seem to be simply a reproductive requirement. Food requirement is another possibility for the motivation of migration. However, even this possibility may not be the primary factor for the migration for the following reasons: 1) In the 1970s, during the course of our study, there were large floating fish enclosures (ikesu), made of nets and containing anchovy for bait of tuna fishing in Koajiro Bay (see Fernholm, 1974). One enclosure contained about 10 tons of fish and the numbers of enclosures varied during the year from a few to about 100. During tuna fishing season, May, June and July, there were many fish enclosures, and many fish in the enclosures died because of the high water temperature and fell down to the bottom of the sea. Thus, food was most abundantly supplied and available for hagfish during May through July, when they migrate out of the Bay. 2) In a pioneer study, Dean (1904) collected hagfish in shallow water almost throughout the year except September and October. This means that hagfish lived in shallow water for a much longer period in the past than during our study, although there were no floating enclosures in days of Dean. Salinity and temperature are known to be limiting factors for all known species of hagfish (Martini, 1998). Indeed, Fernholm (1974) claimed that temperature may provide an important stimulus for migration of E. burgeri. How-ever, temperature is more stable in deeper water, and thus migration from deep to shallow water would not seem to be initiated by temperature and/or salinity. Probably, the integration of several factors, such as food supply, water temperature, and population density, etc., may be the promoting factors for the migration of E. burgeri.

Finally, the present study has demonstrated directly that there are seasonal migrations of the hagfish, E. burgeri, between shallow water of Koajiro Bay and deep water of Sagami Bay, although initiating factors are not known. Spawning sites are considered to be in waters of more than 100 m depth.