Ž Climax depth in the stressed fish was significantly deeper than the control P s 0.016,
. Ž
. n s 5 q 5, U-test . Mean swimming velocity of the reference-provided fish 0.2 mrs
Ž .
was significantly slower than controls 0.28 mrs; P s 0.050, n s 5 q 4, U-test . 3.3. Training
The maximum depth of control fish was 5.4 m, while for conditioned fish, it was 3.3 Ž
. and 3.2 m with or without signal, respectively . Unconditioned fish dived to 7.3 m
depth. The final depths of control fish, conditioned fish, with and without signal, were 1.9, 1.1, and 1.4 m, respectively. Unconditioned fish had a mean 5.3 m final depth; as in
Ž .
some cases, fish did not swim up but stayed near the bottom Fig. 6 . Conversely, conditioned fish constantly stayed in very shallow water when the sound signal was
Ž .
provided Fig. 3 . The mean swimming speed of control fish was 0.33 mrs, while the means for
conditioned fish with and without signal were 0.03 and 0.02 mrs, respectively. There Ž
were significant differences between the control and the conditioned fish P s 0.009 and .
0.014 with and without signal, respectively, U-test . Unconditioned fish showed swim- ming speeds of 0.35 mrs, which was close to the value of the controls.
For controls, no fish remained around the released site in five trials. For conditioned fish, with or without signal, fish remained around the release site in four out of five and
four out of four trials, respectively. Conditioned fish, released with or without signals, Ž
remained at a significantly higher rate than the control P s 0.024 and 0.008, respec- .
tively, Fisher’s test , although there was no significant difference between with and Ž
. without signals P s 0.55 . Unconditioned fish remained around the release site in only
one of five trials.
4. Discussion
Although the maximum swimming depth of released fish differed among trials and treatments of experiments, ranging from 2.0 m for conditioned fish to 12.1 m for
stressed fish, the behaviour of released fish was generally similar. All fish reached a maximum depth within about 30 s and swam up, then staying at a final depth, with only
Ž .
Ž a few exceptions Fig 7 . This confirmed our previous releasing experiment Masuda et
. al., 1993a , which showed that fish released from a 101 bucket reached maximum depths
ranging from 5 to 20 m depth, then swam up to near the surface and stayed at about 3 m depth.
In the fish size experiment, large fish reached deeper maximum depths than small fish, which may reflect a difference in swimming ability, but not a difference in
behavioural characteristics. In most cases, fish in both size groups reached a maximum depth at about 30 s after release and then swam up later. The depth may also reflect a
reaction difference to the releasing stress. Deeper maximum depth, however, has a higher potential for fish to stray from the release site under the same visibility condition.
As 60 mm striped jack suffer little predation risk, the smaller fish may be suitable for releasing.
Fig. 7. Schematic diagram of the results. Fish of unsuitable size or after handling stress tend to dive deeper and stray away from the release site, while fish trained beforehand and released gently under a proper reference
structure tend to dive shallower and stay at the release site.
We compared only two sizes and there remains the possibility that larger sizes of fish could behave differently. According to field observations, wild striped jack shift their
Ž .
habitat from shallow to offshore reef at about 150 mm Masuda et al., 1993b . We do not know if hatchery-reared and released fish may have an innate drive to migrate
offshore when they attain a certain size. Therefore, to prevent straying of large fish, their developmental changes in behaviour should be understood through laboratory experi-
ments.
The stress and vertical reference experiments clearly showed that stressed fish dive deeper and will stray, while a vertical reference structure is effective in preventing this.
Ž .
The stress we used air exposure is similar to the routine handling process of scooping fish by hand net or gathering fish in a net cage. These results strongly suggest that such
handling processes can be a major reason for missing or straying released fish. Ž
. Olla and Davis 1989 examined the effect of air-exposure stress on the vulnerability
to predation in coho salmon. They found that when coho encountered predators within 1 and 60 min after the stress, they were captured more than controls, whereas coho given
90 and 240 min to recover from stress survived predation as well as controls. Their findings suggest that a relatively short period of recovery may greatly improve the
behavior of fish after release.
Ž .
In our previous experiment Masuda et al., 1993a , we released from a 101 bucket. This method might be stressful for the fish and resulted in a relatively deep maximum
Ž .
swimming depth 5–20 m: equivalent to the stress condition in the present study . Therefore, release from a bucket is not suitable for practical use to prevent the straying
of fish. Release after a period of acclimation and with little disturbance is recommended.
The vertical reference treatment resulted in a more shallow maximum swimming depth for released fish than the other two sets. Our results are consistent with the field
Ž .
experiment by Hunter and Mitchell 1968 , who reported that tent-shaped flotsam was much more effective in attracting juvenile fish, predominantly carangid fish, compared
to simple horizontal or vertical flotsam. Relatively complicated structures should work best as a platform for striped jack ranching.
In our conditioning experiment, fish were released from large net-cage releasers, which should have worked as references. In this experiment, conditioned fish dived only
a little and stayed in shallow water, their swimming speed was slow, and they remained at the release site longer compared to control fish. These results suggest that the training
procedure is effective in mitigating stress after release and results in successful marine ranching. Trained fish remained whether a sound signal was present or absent at release.
In the training process, we tried to condition the fish to sound, reference and feeding, but these results suggest that once this conditioning was accomplished, the presence of a
reference can work as a stimulus for fish to remain at the released site.
Ž .
Tsukamoto et al. 1995 compared the learning ability of striped jack at different sizes: 100, 160 and 210 mm. They showed that 100-mm-sized fish learned more quickly
and lost memory more slowly than the other two sizes. As striped jack ranching utilizes the fish’s learning ability, large fish are not suitable in this respect.
Association behaviour is essential for striped jack ranching; thus, we trained these fish to associate with the platform. Critical factors of stock enhancement differ among
Ž .
species. Indeed, Olla and Davis 1989 considered predator avoidance as critical in coho salmon and they succeeded in training them to avoid cod.
5. Conclusion
