Survival of Japanese whiting Sillago japonica and by-catch species captured by a sweeping trammel net

FISHERIES SCIENCE 2001; 67: 21–29

INTRODUCTION

Sweeping trammel net (‘kogi-sashi-ami’) is atypical entangling gear with the encircling andsweeping procedure on sandy coastal fishingground. It is employed to catch demersal speciessuch as red horse head Branchiostegus japonicusand Japanese whiting Sillago japonica as targetspecies. Historically, this fishing gear was intro-duced to Ishikawa Prefecture around 1955 for targeting red horse head. Subsequently, it haddeveloped in Wakasa Bay, and then spread widelyalong the coast of San-in district and some otherareas, such as Niigata and Ehime Prefectures.1 InTateyama Bay, Chiba Prefecture, capturing Japan-ese whiting using the gear in a rather smaller scalehad begun around 1965.

Since the sweeping trammel net became popu-lar as an extremely effective gear for catching

red horse head with an increasing awareness on over fishing, many studies for the efficient re-source management were conducted for red horsehead.2–5 The fish captured by this gear in TateyamaBay were varied in species and sizes, while themain target is Japanese whiting including small-sized immature fish of less than 160 mm totallength.6,7 The by-catch accounted for approxi-mately 18% of total catch per boat during 1-dayoperation trip. Generally, the by-catch species werereleased to the sea after sorting onboard or dis-carded after their removal from the net, eventhough they include commercially important juvenile species.6

In recent years, research on survival of fish afterescape from fishing gear or post release hasbecome increasingly important for establishingsustainable fisheries.8–11 Improving fishing gearselectivity is the first option for the better resourcemanagement, and second establishing the effec-tive release method as an alternative option shouldbe considered if the fishing gear selectivity is low.Because the sweeping trammel net selectivity forthe Japanese whiting was rather low, especially for

Original Article

Survival of Japanese whiting Sillago japonica and by-catch species captured by a sweeping trammel net

ARI PURBAYANTO,1 ATSUHIRO TSUNODA,2 SEIJI AKIYAMA,2,* TAKAFUMI ARIMOTO2

AND TADASHI TOKAI2

1Faculty of Fisheries and Marine Science, Bogor Agricultural University, Bogor 16680,Indonesia and 2Department of Marine Science and Technology, Tokyo University of Fisheries,Konan, Minato, Tokyo 108-8477, Japan

SUMMARY: The survival of Japanese whiting Sillago japonica and by-catch species captured bya sweeping trammel net was examined to assess the effects of differing species, captured condi-tions, and body lengths on survival, in Tateyama Bay, Chiba Prefecture, Japan. All the live sampleswere kept in laboratory tanks for 4 days of successive survival observation. The survival ratio of theJapanese whiting varied by the captured conditions. The fish captured by pocketed condition was27% surviving at the fourth day, which was significantly higher than the gilled and entangled fish.Most of the by-catch species survived for 4 days. About 60% of dead Japanese whiting were observedto have scar injury and damage on the body surface. For the by-catch species, the body damageoccurred only in stripedfin goatfish Upeneus bensasi, Temminck’s surfperch Ditrema temmincki, andtidepool gunnel Pholis nebulosa. Therefore, it suggests that the conservation measure through catch-and-release may be an appropriate method for most by-catch species, but not for the Japanesewhiting.

KEY WORDS: by-catch, captured condition, Japanese whiting, survival, sweeping trammelnet.

*Corresponding author: Tel: 81-3-5463-0470. Fax: 81-3-5463-0360. Email: [email protected]

Received 15 November 1999. Accepted 2 August 2000.

long, comprising two main parts (i.e. body net anddragging net) (Fig. 1). The body net consists of sixpanels of triple-walled net (two outer nets and oneinner net), measuring 18 m in length and 0.75 m indepth when stretched for every panel. Mesh size ofthe inner net was 27.5 mm and the outer net was300 mm. The dragging net was a single net panelwith 30 mm mesh size of thicker mesh twine.6,12

Fishing operation trip for fish sampling wascarried out in Tateyama Bay, Chiba Prefectureduring the period from April to May 1996 and 1997.The fishing operations were done from aroundsunrise to around noon, which was usual opera-tion time of the commercial fishing. The operationduration of sampling trips ranged from 2 h 13 minto 4 h 11 min according to the numbers of opera-tion repetition to take fish required. The averagetime taken for completing one operation (setting,sweeping and hauling) was 19 ± 3 min, and theduration of sweeping and hauling process took6 ± 1 min and 10 ± 3 min, respectively. The numberof successful net hauls varied from seven to 12times for each fishing trip (Table 1).

Sampling and classification of captured condition

Fish caught in a sweeping trammel net weresampled onboard during the hauling process byremoving them carefully from the net. It took lessthan 2 min to remove the individual from the net.The fish samples were then divided by the capturedconditions and put into the plastic containers of 50 L capacities at density of less than 20 indivi-duals per container. Seawater was continuouslyexchanged. The fish conditions kept in the plasticcontainers were checked, while dead ones were

large fish in comparison to gill net,12 it is necessaryto validate the catch and release method especiallyfor the by-catch species, through carrying out aseries of experiments designed to identify andquantify the level of the released mortality.

The survival studies previously done, however,have been limited to a few fishing methods9 andmost of them were concentrated on the fish escap-ing from the trawl cod-end.10 Very few studies havebeen done about mortality associated with catchand release of fish from nets such as gill nets andtrammel nets.13 There are two methods usuallyemployed to observe the survival of fish (i.e. bykeeping the fish in the net-pen (cage) set on the seabottom,14–18 and by keeping them in laboratorytanks for an appropriate time duration.19,20 Wassen-berg and Hill summarized several factors affectingthe survival of fish (e.g. decrease in pressure as thefish is brought up to the surface, exposure durationin air on deck, and damage due to the fishing gearcontact).20 In addition, the proportion of survival intrawl fishery varies among taxonomic groups andeven among species within a genus.21

In the present study, to investigate the survivalof Japanese whiting Sillago japonica and other by-catch species captured by a sweeping trammel net,we conducted experiments in the laboratory tankto assess the effect on the different species, cap-tured conditions, and body lengths associated withthe survival after capture.

MATERIALS AND METHODS

Fishing gear and fishing operation

The sweeping trammel net used in the presentexperiment was a commercial fishing unit of 118 m

22 FISHERIES SCIENCE A Purbayanto et al.

Fig. 1 Schematic view of a sweeping trammel net (triple-walled net) used in the experiment showing fish captured byentangled and pocketed conditions.

removed immediately, counted and their totallengths were measured. Water temperature wasalso recorded, ranging from 15.0 to 17.8°C throughthe sampling period.

All the fish samples were immediately trans-ported to the Banda Marine Laboratory of TokyoUniversity of Fisheries by the fishing boat. It tookabout 40 min to transport the fish from the sampling location to the laboratory. Immedi-ately after arriving at the laboratory, they were kept in two units of rectangular fiberglass tanks(1.8 ¥ 0.9 ¥ 0.7 m) containing seawater of 60 cmdeep for survival observation. Each tank was parti-tioned into two with stiff plastic nets to keep thefish samples according to the captured conditions.To maintain water quality, fresh seawater was con-tinuously supplied into the tanks.

The captured conditions were classified intofour groups on the basis of the state of fish retainedin the net and defined as follows:6,22–25 (i) snagged:when the fish is held by a mesh along the head partfrom the mouth to the pre-operculum; (ii) grilled,when the fish is held by a mesh along the head partfrom the operculum to the frontal dorsal fin; (iii)entangled, when one part of the fish body (teeth,maxillaries, fins or other projections) is held by netting, or entirely the fish body is wrappedtightly by netting (referred as complicated en-tanglement, which commonly occurred in theJapanese whiting); and (iv) pocketed, when the fishentrapped in the net with the formed pocket by the small-meshed netting between the two big-meshed walls (Fig. 1).

Survival observation

Several previous studies on the survival test of cap-tured fish reported that most of the animals died

within 48 h after release,13 and Wassenberg and Hillsuggested that 4 days observation was the mostsuitable minimum duration to measure the sur-vival of animals discarded from trawl catch.20 In thepresent experiment, similarly, the survival of thefish was observed up to 4 days. During this obser-vation, the dead fish were removed, counted andtheir total lengths were measured. On the fourthday, the survivors were counted, their total lengthmeasured, and visually observed for an extendedinjury or damage to their bodies. The dead fishwere also visually examined for damaged bodyappearance so that they were classified on thebasis of their damaged pattern and degree. Theaverage water temperatures recorded in the tankswere 17.4 ± 0.3°C in 1996 and 15.3 ± 0.4°C in 1997.The fish were not fed during the experiment.

Data analysis

We calculated the survival ratio of Japanesewhiting for each captured condition and lengthsize group. To analyze the effect of length size onthe survival of Japanese whiting, all fish samplesregardless of the captured conditions were classi-fied into three groups by length sizes (i.e. small(< 160 mm total length), medium (160–200 mm),and large-sized fish (> 200 mm).6 The effect oflength size itself on the survival was analyzed usingthe data of pocketed fish samples, only for the middle- and large-sized group because of their sample numbers in total compared with theother captured conditions. The small-sized group,however, was excluded from this analysis becauseof its inadequate sample number.

The survival ratio at observation time-t was calculated as the proportion of fish surviving attime-t to the total number of fish initially sampled

Survival of fish captured by a sweeping trammel net 23

Table 1 Characteristics of fishing operation and captured conditions, and water temperature during sampling

No. Date No. Fishing No. samples by Waternet operation duration (min) captured conditions temperature (°C)

nS nG nE nP (M, L)

Japanese whiting1. 24 April 96 8 251 10 17 14 15 (11, 4) 16.52. 16 May 96 7 146 19 66 20 28 (26, 2) 17.83. 23 April 97 10 211 – – – 15 (11, 4) 15.04. 29 April 97 12 223 24 – 11 26 (17, 9) 15.05. 1 May 97 7 133 – – – 25 (16, 9) 15.6By-catch1. 24 April 96 8 251 1 5 8 2 16.52. 16 May 96 7 146 1 13 19 9 17.8

nS, snagged; nG, gilled; nE, entangled; nP (M, L), pocketed (medium-, large-sized group).

Fish condition after capture and physical injury and damage

Most of the sampled fish tended to swim calmlywith hyperactive breathing in the upper layer of theplastic container on the deck, while the large-sizedJapanese whiting were observed to be weakerbecause several numbers of them swam upside

according to the captured condition and the lengthsize for each sampling period (Table 1). The samplenumber for Japanese whiting by length size wasobtained from rearrangement of the initial sam-ples into the length size group. In the case of by-catch species, the pooled data obtained from twofishing trips were used due to the small number of individuals for each species.

The observation time-t was measured asdecimal fractions of days elapsed after capture (i.e.at 12 h interval counted from the end time offishing operation (referred to as t = 0)).

The survival curves for Japanese whiting wereestimated from the mean of observed survivalratios according to the captured condition andlength size. In this estimation, the dead fish duringsampling was counted together with the number ofdead fish in the first 12 h, and then its observed survival ratio was used for estimating the sur-vival curve. The survival ratio at the obser-vation time-t can be described by the following model:26

where is the indefinite survival ratio and k is the slope of survival curve. For fitting the model,the parameters and k were estimated using thenon-linear least square method. Significant differ-ences (a = 0.05) between the mean values of mor-tality during sampling and between the meanvalues of survival ratios for Japanese whiting at day4 were examined using Wilcoxon Mann–Whitneytest.27

RESULTS

Length size distribution of fish samples

The total 290 individuals of Japanese whiting and 58 individuals in nine by-catch species weresampled from the 44 successful operations.According to the captured condition, almost all ofthe gilled fish were small sized, while the fish largerthan 230 mm were all caught by pocketed condi-tion (Fig. 2). Means and standard errors in totallength were 178 ± 19 mm for the fish captured bysnagged, 147 ± 12 mm for gilled, 173 ± 20 mm forentangled, and 192 ± 23 mm for pocketed fish.

The range of total length for by-catch variedamong the species (Table 2). Among nine by-catchspecies, two species of olive flounder Paralichthysolivaceus (53–127 mm), red sea bream Pagrusmajor (92–115 mm), and bluefin searobin Cheli-donichthys spinosus (138–252 mm) were commer-cially important.

S•

S•

ˆ ,S S S et

kt( ) • •

-= + -( )1

S t( )


Fig. 2 Length frequency distribution of Japanesewhiting by captured conditions.

down with their swim bladders swelled. Duringtransportation, almost all the fish swam calmlynear the surface, while a few fish lay upside downon the bottom. The fish continued to swim calmlyafter keeping in the laboratory tanks within 1 day.From day 2 some fish showed active swimmingnear the bottom of the tanks.

Approximately 60% of dead Japanese whitingwere observed to have scar injury and damage onthe body surface. According to the damage pat-terns, the dead Japanese whiting could be classi-fied into six categories as follows (Fig. 3): (i) minorscale loss on the upper parts of fish head in frontof dorsal fin; (ii) scar injuries and minor scale lossaround the circumference of the maximum bodygirth; (iii) scale loss on several parts of the fishbody; (iv) scale loss on several parts of the fish bodyand the fish head; (v) extended injury on the headand extinction of fins; (vi) extended injury on thehead and major fish body, and extinction of fins. Atotal of 83% of snagged fish and 70% of the pock-eted fish which died between day 0 and 1 occupiedpattern 1, while 81% of the gilled fish and 73% ofthe entangled fish occupied pattern 4. Most fishthat died between days 2 and 4 occupied patterns5 and 6, which exhibited the extended injurybodies and the extinction of fins (Table 3). A few ofthe Japanese whiting that survived over 4 days

of the experimental duration were observed toexhibit an extended injury and damaged body.

Scale loss and damaged bodies for the by-catchspecies occurred around the circumference ofmaximum body girth only in stripedfin goatfishUpeneus bensasi, Temminck’s surfperch Ditrematemmincki, and tidepool gunnel Pholis nebulosabecause they were gilled.

Survival of the Japanese whiting

Some Japanese whiting died immediately afterbeing kept in the containers onboard during sam-pling. The mortality ratios of gilled fish tended tobe higher than the others, however, these were notsignificantly different among the fish samples ofdifferent captured conditions (mortality for thesnagged, gilled, entangled, and pocketed fish were35.70 ± 5.39%, 25.98 ± 9.31%, 29.22 ± 11.81%, and22.07 ± 6.09%, respectively).

The plots of survival ratio against the dayselapsed after capture from days 0 to 4 and the fittedsurvival curves by the captured conditions and the body lengths are shown in Fig. 4 and Fig. 5,respectively. Figure 6 shows the survival curves for medium- and large-sized pocketed Japanesewhiting. Parameter estimates of those fitted curves


Table 2 Range of total length and samples number (n) of nine by-catch species

No. Species Range of total length (mm) Samples (n)

1. Japanese filefish Rudarius ercodes 47–71 162. Bluefin searobin Chelidonichthys spinosus 138–252 83. Stripedfin goatfish Upeneus bensasi 113–182 74. Temminck’s surfperch Ditrema temmincki 79–94 65. Olive flounder Paralichthys olivaceus 53–127 66. Grass puffer Takifugu niphobles 105–147 57. Richardson’s dragonet Repomucenus richardsonii 182–206 48. Red sea bream Pagrus major 92–155 39. Tidepool gunnel Pholis nebulosa 205–233 3

Table 3 Percentage of the dead Japanese whiting by damaged patterns and captured conditions

Patterns* Death from day 0 to 1 Death from day 2 to 4S(6) G(16) E(15) P(10) S(8) G(13) E(8) P(8)

1 83 – 20 70 – – 12 –2 – 19 – – – 16 – –3 – – 7 30 – – – 114 17 81 73 – 13 15 25 –5 – – – – 75 23 – 506 – – – – 12 46 63 39

Values in parentheses represent number of the dead fish to be observed with injury or damage on the body’s surface.S, snagged; G, gilled; E, entangled; P, pocketed.* The damaged patterns can be seen in Fig. 3.

and both the estimated and observed survivalratios at day 4 are shown in Table 4.

The survival curves steeply decreased within 2days and then tended to remain stable from day 3 for all captured conditions (Fig. 4), except for the entangled fish, which continuously decreasedapproaching the minimum value of indefinite sur-vival ratio ( was 0.02). The survival curves for thepocketed, snagged and gilled fish had a similardescending pattern of being flat from day 1 to day4 and the slope (k) of the curves ranged from 1.87to 2.27 (Table 4). On the other hand, the survivalcurve of entangled fish had the lowest k-value of1.27. Therefore, the survival ratio of pocketed fishwas higher than those of the snagged, gilled, and

S•


Fig. 3 Six damaged body patterns of the dead Japanese whiting. (a) Minor scale loss on the upper parts of fish headin front of dorsal fin; (b) scar injuries and minor scale loss around the circumference of the maximum body girth; (c) scale loss on several parts of the fish body; (d) scale loss on several parts of the fish body and the fish head; (e) extended injury on the head and extinction of fins; and (f ) extended injury on the head and major fish body, andextinction of fins.

Fig. 4 Survival curves for Japanese whiting by capturedconditions. Marks indicate the observed survival ratios.

Fig. 5 Survival curves for Japanese whiting by lengthsizes. Marks indicate the observed survival ratios.

Fig. 6 Survival curves for middle (160–200 mm) andlarge-sized (>200 mm) pocketed Japanese whiting.Marks indicate the observed survival ratios.

(a) (b) (c)

(d) (e) (f )

entangled fish. Furthermore, its survival ratio atday 4 (27.15 ± 4.32%) was significantly differentfrom the gilled and entangled fish.

The survival curves by length sizes shown inFig. 5 revealed that the large- and medium-sizedfish had a similar descending pattern because thek-values of the fitted curves were almost the sameas shown in Table 4 (i.e. 1.42 and 1.43 for the large-and medium-sized fish, respectively). In contrastto the small-sized fish, the survival curve showedthe steep decrease up to day 1 and remained stableflat after day 1, compared to those curves for themiddle- and large-sized fish (the k-value was 2.76).The survival ratios for the large- and medium-sizedfish showed a similar tendency, which was higherthan the small-sized fish.

The survival curve for the middle-sized pock-eted fish seemed rather steep up to day 2 in com-parison to the large-sized fish, and this conditionreversed from day 2 to day 4 due to the higher mor-tality of the large fish (Fig. 6). The survival ratios,however, were not significantly different amongthose fish. The observed survival ratios at day 4were 30.72 ± 5.07% for the middle-sized fish and26.11 ± 8.13% for the large-sized fish.

Survival of the by-catch species

No mortality of the by-catch species occurredimmediately after keeping in the plastic containeronboard during sampling, except for stripedfingoatfish Upeneus bensasi and Temminck’s surf-perch Ditrema temmincki of which mortality ratioswere 28.57% and 66.67%, respectively. Most of theby-catch species survived for over 4 days aftercapture. Three species of them (Japanese filefish

Rudarius ercodes, grass puffer Takifugu niphobles,and red sea bream Pagrus major) showed 100%surviving over the observed duration. The sur-vival ratios of other species at day 4 was 50% forRichardson’s dragonet Repomucenus richardsonii,83.33% for olive flounder Paralichthys olivaceus,and 87.50% for bluefin searobin Chelidonichthysspinosus. Stripedfin goatfish, Temminck’s surf-perch, tidepool gunnel Pholis nebulosa showed low survival ratios of 14.29%, 16.67% and 33.33%,respectively.

DISCUSSION

The present experiment results clarified that mostof the by-catch species represented high survivalratios in comparison to the Japanese whiting. It isevident that the by-catch species were relativelyrobust to the body damage due to the gear contact,except for stripedfin goatfish Upeneus bensasi andTemminck’s surfperch Ditrema temmincki. Thesespecies showed low survival ratios (the survivalratios at day 4 were 14.29% for stripedfin goatfishand 16.67% for Temminck’s surfperch) similar tothe snagged and pocketed Japanese whiting (thesurvival ratios were 17.95% for the snagged fish and27.15% for the pocketed fish). The hard spines andfirm scale construction of most of the by-catchspecies probably reduce the influence of the gearcontact to the body damage, while two by-catchspecies mentioned above seem to have a fragilescale construction similar to the Japanese whiting.

The survival of Japanese whiting was greatlyaffected by the captured condition, showing a sig-nificant difference in the survival ratio among thefish caught by four different conditions. We con-


Table 4 Estimates of the parameters and standard errors (in parentheses) from fits of the survival curves of Japanesewhiting and the estimated (S(t=4)) and the observed survival ratios (S(t=4)) at day 4

Analyzed components S∞ k S(t=4) S(t=4)

by sample groups

Capture conditionsSnagged 0.22 (0.09) 2.16 (0.87) 22.62 ± 8.81 17.95 ± 7.69Gilled 0.10 (0.04) 2.28 (0.25) 9.60 ± 3.85 6.79 ± 4.97Entangled 0.02 (0.01) 1.27 (0.41) 4.92 ± 1.59 4.70 ± 2.63Pocketed 0.31 (0.06) 1.87 (0.78) 31.79 ± 5.32 27.15 ± 4.32*

Length size for all fishSmall size 0.13 (0.03) 2.76 (0.31) 12.73 ± 3.01 10.81 ± 3.69Middle size 0.24 (0.09) 1.43 (0.31) 24.79 ± 7.96 21.95 ± 6.76Large size 0.20 (0.04) 1.42 (0.76) 25.65 ± 3.13 20.28 ± 0.28

Length size for pocketed fishMiddle size 0.30 (0.03) 1.79 (0.78) 33.97 ± 4.83 30.72 ± 5.07Large size 0.38 (0.25) 1.95 (1.02) 26.70 ± 9.63 26.11 ± 8.13

The survival ratios are represented by mean ± SE.* Significantly different from the gilled and entangled fish (a = 0.05).

sized fish. It is considered that the length size offish has relationship with the captured conditionbecause of the inclusion of different captured con-ditions in the analysis. In this case, the capturedcondition in sweeping trammel net dependsgreatly on the length size of Japanese whiting. Thelarge-sized fish, therefore, tended to be capturedmostly by pocketed condition, while the small-sized fish tended to be captured mostly by thegilled condition (Fig. 2). In order to clarify whetherthe length size itself affects the survival of Japanesewhiting or not, the survival curves for middle- andlarge-sized pocketed fish (Fig. 6) can be used forthe next explanation. It suggested that the differentlength size in Japanese whiting did not affect thesurvival results because the survival ratios betweenthe medium- and the large-sized pocketed fishwere not significantly different. We consider thatthe fish, which were caught in similar conditions,would receive relatively similar degrees of effect inphysical injury or damage.

We could not clarify whether the captured condition influences the survival ratios of the by-catch species, it can be discussed that the influ-ences may be significant for three such species(stripedfin goatfish, Temminck’s surfperch, andtidepool gunnel). These species showed lower sur-vival ratios than the others as the result of the bodydamage due to the gear contact. However, furtherclarification is needed using a large sample data.

With the evidence of no extended physicaldamage of the surviving by-catch species at day 4 after capture, we expect they will continue tosurvive. Therefore, we suggest that the conserva-tion measure through catch and release may be anappropriate method for the by-catch species, espe-cially for young fish of commercially importantspecies such as olive flounder Paralichthys oli-vaceus, red sea bream Pagrus major, and bluefinsearobin Chelidonichthys spinosus. However, this may not be effective for the Japanese whitingbecause most of the small fish, which were captured by gilled conditions, died during the 4 days survival experiment. The conservation ofthe small-sized Japanese whiting, which comprisesome immature fish,7 can be done through themesh selectivity improvement of the sweepingtrammel net.12 The success of the selectivityimprovement as the conservation measure,however, greatly depends on the survival of fishafter escape from the net.

ACKNOWLEDGMENTS

The authors wish to thank Mr. K. Itoh for allowingus to sample using his fishing boat. Our thanks are

sider that the captured conditions give differentinfluences on the degree of physical injury anddamage incurred during capturing process. Thephysical damage may result in the death of fish,28

and is a major cause of mortality.29 Beek et al.reported that damage conditions of plaice Pleu-ronectes platessa and sole Solea solea discarded bytrawl caused a different survival ratio.29 The obser-vation result on the dead Japanese whiting indi-cated that the gilled and entangled fish sufferedheavily from injuries and damage to their bodiescompared to the snagged and pocketed fish asshown in Table 3. The gilled and entangled fishexhibited scar injuries around their gill cover andscale losses in the majority of bodies due to thetight contact with the net twine as a result ofviolent struggling during the fish retained in thenet.30 In addition, the hauling process that mayincrease the net tension can be considered as the other factor to attribute to the body injury ordamage.

Even with the minor injury, many snagged andpocketed Japanese whiting that survived at day 1would die from days 2 to 4. These fish exhibited anextended injury on the head and the extinction offins due to them becoming rotten (75% for thesnagged and 50% for the pocketed fish occupiedpattern 5) as shown in Table 3. The gilled andentangled fish, however, showed more extremelyextended injuries on the head and major fish body, and their fins became extinct (46% for thegilled and 63% for the entangled fish occupiedpattern 6). Main and Sangster reported thathaddock Melanogrammus aeglefinus, which hadescaped through a square mesh cod-end after 2 or3 days kept in the cage, the scale damaged areas ofthe fish eventually turned white in color and more-over their tails slowly began to rot away.14

The conditions mentioned above led to the lowsurvival ratios of the gilled and entangled fish asthe survival curves were sharply decreased, espe-cially for the entangled fish which showed the highmortality (k-value was the lowest of 1.27). Chopinet al. reported that the adaptive behavior was notpossible in fish caught by trammel net in which the degree of entanglement or the constriction ofnetting around the fish body and gills remainedconstant or increased with capture duration due tothe violent struggling.31 The reason of low survivalratios of the gilled and entangled fish are also sup-ported by Tsunoda et al.30 who reported that thegilled Japanese whiting, after capture by sweepingtrammel net, had the higher stress level and longerrecovery time than the pocketed fish.

The survival curves, according to the lengthsizes in Fig. 5, showed that the large- and medium-sized fish had higher survival ratios than the small-


also given to Ms. S. Saito for her valuable assistancethrough her thesis work, and Dr. Yasuyuki Koike,Messrs. M. Masuko, N. Ishikawa and J. Itoh, thestaff members of Banda Marine Laboratory ofTokyo University of Fisheries for their generouscooperation during the experiment. We thank theanonymous reviewers for their constructive cor-rections and comments on the manuscript.

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Survival of Japanese whiting Sillago japonica and by-catch species captured by a sweeping trammel net