No.64 (March,2003)
No.64 (March,2003)
A-365 Shuichi NISHIUCHI
A study on size-selectivity of hair crab pots
The hair crab, Erimacrus isenbeckii, is an important marine resource in Hokkaido. Because the catches have been low in recent years, stock enhancement is becoming more important. In order to establish selective fishing methods corresponding to the minimum legal size, the size-selectivity of various mesh sizes and escape-vents of hair crab pots were estimated based on data from comparative fishing experiments, and the results compared. The sizeselectivity of each mesh size and escape-vent size were estimated using the SELECT method. The selectivity master curves for both mesh size and escape-vent size were estimated using the expanded SELECT model. The results obtained from this study are as follows:
Since some problems have occurred in previous studies on the selectivity of pots (traps), a new method based on the fishing process of hair crab pots was developed using the SELECT model and the master curve method. This method was useful for estimating the size-selectivity for both mesh and escape-vents, and showed that the assumption of equal relative fishing efficiency between hair crab pots with various mesh sizes or escape-vent sizes is not always applicable. Moreover, the assumption that a proportional relationship exists between the rostral length and the mesh size that show the same retention probability was not accepted. In order to compare the mesh selectivity and escape-vent selectivity, equations which represent the relationship between the 50% retention rostral length ( l50) and the selection range ( SR) were derived from the selectivity master curve for both mesh size and escape-vent size. Graphically, the mesh selectivity and escape-vent selectivity can be readily compared.
Mesh selectivity curves for each mesh size and master curve were estimated on the data from comparative fishing experiments in the case of both the 1-day soak and 2-day soak. Mesh sizes used for experiments were 57, 72，87， 102 and 117 mm for the internal stretched length. The logistic function was used to give the mesh selection.
The difference in 50% retention rostral length between soak times was not clear. However, the selection range for the 2-day soak was narrower compared with that for the 1-day soak, indicating that the mesh selectivity would change with the soak time. In the meantime, no difference in the relative fishing efficiencies among mesh sizes was found for the 1-day soak. However, the relative fishing efficiency in the selected models increased with increasing mesh size in the case of the 2-day soak. This indicates that the relative fishing efficiency would change with the soak time. The assumption that a proportional relationship exists between the rostal length and the mesh size that shows the same retention probability is not accepted. From the master curve, the mesh size with which the rostral length of 50% retention corresponding to the minimum legal size of 80 mm and its selection range were calculated as 114 mm and 6.3 mm for the 1-day soak, and 114 mm and 2.5 mm for the 2-day soak, respectively. These results show that the mesh selectivity of hair crab pots are sharp. On the other hand, the rostral length of 50% retention for the current minimum legal mesh size of 115-mm (knot to knot length) were calculated as 78.3-mm for the 1-day soak and 78.4-mm for the 2-day soak, respectively, slightly smaller than the minimum legal rostral length of 80-mm.
In the years after the introduction of the mesh-size restriction, the proportion of hair crabs that were of sublegal- sized crabs caught by commercial vessels was significantly low compared with that for a research vessel in the three areas off Abashiri subprefecture. This indicates the effectiveness of the mesh restriction. However, the percentage of sublegal hair crabs caught by the commercial vessels were still high（30-40% on an average) in the years after the introduction of the mesh-size restriction.
The preliminary experiment using pots with two 75-mm wide × 48-mm high vents and those with two 85-mm wide × 55-mm high vents showed that a significant improvement on the selectivity of hair crab pots can be achieved by incorporating escape-vents into pots. The 85 × 55-mm vents were more efficient than the 75 ×48- mm vents in reducing the catch of sublegal crabs and retention of legal crabs.
The size-selectivity curves for each escape-vent size and master curve using the logistic function were estimated on data from comparative fishing experiments in the cases of both 1-day soak and ≧2-day soak. The pots used for the experiments were control pots without an escape-vent and the other four types of pots with two escape-vents of 100-mm wide and 49, 51, 53 and 55-mm high. For each escape-vent size, the rostral length of 50% retention in the case of the ≧2-day soak was larger than that for the 1-day soak, and the selection range for the ≧2-day soak was narrower than that for the 1-day soak. These results suggest that the size-selectivity of escape-vents vary with the soak time. The optimum escape-vent size that would retain legal-sized crabs but allow sublegal-sized ones to escape was estimated to be 51-mm high. The assumption of equal relative fishing efficiency was accepted for the 1-day soak, while it was not accepted for the ≧2-day soak. The relative fishing efficiency was increased by increasing the escape-vent size for the ≧2-day soak. On the other hand, the principle of geometric similarity of Baranov was accepted for both the 1-day soak and the ≧2-day soak.
The number of escape-vents also affected the size-selectivity of the hair crab pots. While no difference in the sizeselectivity was observed between the number of escape-vents for the 1-day soak, the 50% retention rostral length of two 85 × 55-mm vented pots was larger than that of one 85 × 55-mm vented pots for the ≧2-day soak. Thus the pots with two 85 × 55-mm escape-vents were more efficient than those with one 85 × 55-mm escape-vent in reducing the sublegal crab catch for the ≧2-day soak. From the above results the most efficient escape-vent to be incorporated into pots would be two of 100-mm wide × 51-mm high vents.
With the intent to save cost and labor, the theoretical size-selectivity curves based on the body depth vs. rostral length regression were calculated for the escape-vent sizes used in the experiments. The 50% retention rostral lengths estimated from the SELECT model were apparently lower than those predicted by the theoretical sizeselectivity curves for the 1-day soak, while the theoretical size-selectivity curves were close to the SELECTproduced size-selectivity curves for the ≧2-day soak. This indicates the necessity of introducing the factor relating to the soak time into calculating theoretical curves.
Comparing the SR-l50 relationship between mesh size and escape-vent size proved that the escape-vent selectivity is as sharp as that of the mesh selectivity. There are some advantages of escape-vents compared with mesh (i.e., stable selectivity, simplicity to change the selectivity of pots). Moreover, the selectivity of escape-vents could be ameliorated with some options (e.g., material, shape, number, and the location of escape-vents). Therefore, I concluded that escapevents are an effective method for selective fishing of legal sized hair crabs, Erimacrus isenbeckii.
The hair crab, Erimacrus isenbeckii, is an important marine resource in Hokkaido. Because the catches have been low in recent years, stock enhancement is becoming more important. In order to establish selective fishing methods corresponding to the minimum legal size, the size-selectivity of various mesh sizes and escape-vents of hair crab pots were estimated based on data from comparative fishing experiments, and the results compared. The sizeselectivity of each mesh size and escape-vent size were estimated using the SELECT method. The selectivity master curves for both mesh size and escape-vent size were estimated using the expanded SELECT model. The results obtained from this study are as follows:
Since some problems have occurred in previous studies on the selectivity of pots (traps), a new method based on the fishing process of hair crab pots was developed using the SELECT model and the master curve method. This method was useful for estimating the size-selectivity for both mesh and escape-vents, and showed that the assumption of equal relative fishing efficiency between hair crab pots with various mesh sizes or escape-vent sizes is not always applicable. Moreover, the assumption that a proportional relationship exists between the rostral length and the mesh size that show the same retention probability was not accepted. In order to compare the mesh selectivity and escape-vent selectivity, equations which represent the relationship between the 50% retention rostral length ( l50) and the selection range ( SR) were derived from the selectivity master curve for both mesh size and escape-vent size. Graphically, the mesh selectivity and escape-vent selectivity can be readily compared.
Mesh selectivity curves for each mesh size and master curve were estimated on the data from comparative fishing experiments in the case of both the 1-day soak and 2-day soak. Mesh sizes used for experiments were 57, 72，87， 102 and 117 mm for the internal stretched length. The logistic function was used to give the mesh selection.
The difference in 50% retention rostral length between soak times was not clear. However, the selection range for the 2-day soak was narrower compared with that for the 1-day soak, indicating that the mesh selectivity would change with the soak time. In the meantime, no difference in the relative fishing efficiencies among mesh sizes was found for the 1-day soak. However, the relative fishing efficiency in the selected models increased with increasing mesh size in the case of the 2-day soak. This indicates that the relative fishing efficiency would change with the soak time. The assumption that a proportional relationship exists between the rostal length and the mesh size that shows the same retention probability is not accepted. From the master curve, the mesh size with which the rostral length of 50% retention corresponding to the minimum legal size of 80 mm and its selection range were calculated as 114 mm and 6.3 mm for the 1-day soak, and 114 mm and 2.5 mm for the 2-day soak, respectively. These results show that the mesh selectivity of hair crab pots are sharp. On the other hand, the rostral length of 50% retention for the current minimum legal mesh size of 115-mm (knot to knot length) were calculated as 78.3-mm for the 1-day soak and 78.4-mm for the 2-day soak, respectively, slightly smaller than the minimum legal rostral length of 80-mm.
In the years after the introduction of the mesh-size restriction, the proportion of hair crabs that were of sublegal- sized crabs caught by commercial vessels was significantly low compared with that for a research vessel in the three areas off Abashiri subprefecture. This indicates the effectiveness of the mesh restriction. However, the percentage of sublegal hair crabs caught by the commercial vessels were still high（30-40% on an average) in the years after the introduction of the mesh-size restriction.
The preliminary experiment using pots with two 75-mm wide × 48-mm high vents and those with two 85-mm wide × 55-mm high vents showed that a significant improvement on the selectivity of hair crab pots can be achieved by incorporating escape-vents into pots. The 85 × 55-mm vents were more efficient than the 75 ×48- mm vents in reducing the catch of sublegal crabs and retention of legal crabs.
The size-selectivity curves for each escape-vent size and master curve using the logistic function were estimated on data from comparative fishing experiments in the cases of both 1-day soak and ≧2-day soak. The pots used for the experiments were control pots without an escape-vent and the other four types of pots with two escape-vents of 100-mm wide and 49, 51, 53 and 55-mm high. For each escape-vent size, the rostral length of 50% retention in the case of the ≧2-day soak was larger than that for the 1-day soak, and the selection range for the ≧2-day soak was narrower than that for the 1-day soak. These results suggest that the size-selectivity of escape-vents vary with the soak time. The optimum escape-vent size that would retain legal-sized crabs but allow sublegal-sized ones to escape was estimated to be 51-mm high. The assumption of equal relative fishing efficiency was accepted for the 1-day soak, while it was not accepted for the ≧2-day soak. The relative fishing efficiency was increased by increasing the escape-vent size for the ≧2-day soak. On the other hand, the principle of geometric similarity of Baranov was accepted for both the 1-day soak and the ≧2-day soak.
The number of escape-vents also affected the size-selectivity of the hair crab pots. While no difference in the sizeselectivity was observed between the number of escape-vents for the 1-day soak, the 50% retention rostral length of two 85 × 55-mm vented pots was larger than that of one 85 × 55-mm vented pots for the ≧2-day soak. Thus the pots with two 85 × 55-mm escape-vents were more efficient than those with one 85 × 55-mm escape-vent in reducing the sublegal crab catch for the ≧2-day soak. From the above results the most efficient escape-vent to be incorporated into pots would be two of 100-mm wide × 51-mm high vents.
With the intent to save cost and labor, the theoretical size-selectivity curves based on the body depth vs. rostral length regression were calculated for the escape-vent sizes used in the experiments. The 50% retention rostral lengths estimated from the SELECT model were apparently lower than those predicted by the theoretical sizeselectivity curves for the 1-day soak, while the theoretical size-selectivity curves were close to the SELECTproduced size-selectivity curves for the ≧2-day soak. This indicates the necessity of introducing the factor relating to the soak time into calculating theoretical curves.
Comparing the SR-l50 relationship between mesh size and escape-vent size proved that the escape-vent selectivity is as sharp as that of the mesh selectivity. There are some advantages of escape-vents compared with mesh (i.e., stable selectivity, simplicity to change the selectivity of pots). Moreover, the selectivity of escape-vents could be ameliorated with some options (e.g., material, shape, number, and the location of escape-vents). Therefore, I concluded that escapevents are an effective method for selective fishing of legal sized hair crabs, Erimacrus isenbeckii.
A-366 Keizo YOSHIMURA and Masami KAWASHITA
Comparison of body proportions and meristic characters betweenhatchery-reared and wild Japanese flounder juveniles
This article reports the morphological characteristics in hatchery-reared Japanese flounder, Paralichthys olivaceus, based on analysis of the seedlings produced by Haboro station of Hokkaido aquaculture promotion corporation in 1996, comparing with the wild juveniles collected from northern coast of Hokkaido during 1995 to 1996.
The hatchery-reared flounders had relatively deeper body and larger head than wild juveniles in equal standard length; resulting rather rhombic or round body shape in lateral aspect, against the stream-lined shape of the wild juveniles. The vertebrae of reared flounder shifted the mode to 37 in total counts versus 38 in wild, and were often deformed with fused vertebra. Though such anomaly and numerical reduction did increase the degree of body depth/length ratio particularly in the cases of heavy deformation, the relative shortening of body occurred independently of the anomaly or reduction. It was suggested that an allometric deviation was induced by rearing conditions in which the warmer water temperature and food nutrition play the important roles.
The numbers of dorsal and anal fin ray of the reared flounder were not significantly different from the wild juveniles and were correlated to vertebral numbers. It was considered that the number of fin rays are subordinative to the vertebral number, consequently the decrease of vertebral number offset the increase of fin rays which is often observed in the rearing with warmed water.
This article reports the morphological characteristics in hatchery-reared Japanese flounder, Paralichthys olivaceus, based on analysis of the seedlings produced by Haboro station of Hokkaido aquaculture promotion corporation in 1996, comparing with the wild juveniles collected from northern coast of Hokkaido during 1995 to 1996.
The hatchery-reared flounders had relatively deeper body and larger head than wild juveniles in equal standard length; resulting rather rhombic or round body shape in lateral aspect, against the stream-lined shape of the wild juveniles. The vertebrae of reared flounder shifted the mode to 37 in total counts versus 38 in wild, and were often deformed with fused vertebra. Though such anomaly and numerical reduction did increase the degree of body depth/length ratio particularly in the cases of heavy deformation, the relative shortening of body occurred independently of the anomaly or reduction. It was suggested that an allometric deviation was induced by rearing conditions in which the warmer water temperature and food nutrition play the important roles.
The numbers of dorsal and anal fin ray of the reared flounder were not significantly different from the wild juveniles and were correlated to vertebral numbers. It was considered that the number of fin rays are subordinative to the vertebral number, consequently the decrease of vertebral number offset the increase of fin rays which is often observed in the rearing with warmed water.
A-367 Nobukazu SATOH , Takashi FUJIOKA and Yohei SHIMIZU
Ingestion of live food by the larvae of Brown Sole Pleuronectes herzensteini at different temperatures in the Hokkaido region
This experiment was conducted to evaluate the changes with the passage of time in the number of rotifer and Artemia nauplii ingested , the survival rate , the growth, the rate of pigmentation, and the rate of normal eye position at two different temperatures (15℃and 18℃) of the larval brown sole Pleuronectes herzensteini.
The dispersion for the survival rate (51days after hatching )of the group at 18 ℃ (group 18℃)was greater than that of 15 ℃ (group15℃) . On the other hand , the rates for normal pigmentation and eye position , there were not significantly different. The time-related changes in the number of live food ingested by larval brown sole at 18 ℃ was usually found to be less than that for group at 15 ℃. From the developmental stage D to E , the number of rotifer ingested by fish decreased,and this tendency was most remarkable in group 18℃.
These results shows that it is more desirable to raise larval brown sole in Hokkaido region at 15 ℃ than at 18℃.
This experiment was conducted to evaluate the changes with the passage of time in the number of rotifer and Artemia nauplii ingested , the survival rate , the growth, the rate of pigmentation, and the rate of normal eye position at two different temperatures (15℃and 18℃) of the larval brown sole Pleuronectes herzensteini.
The dispersion for the survival rate (51days after hatching )of the group at 18 ℃ (group 18℃)was greater than that of 15 ℃ (group15℃) . On the other hand , the rates for normal pigmentation and eye position , there were not significantly different. The time-related changes in the number of live food ingested by larval brown sole at 18 ℃ was usually found to be less than that for group at 15 ℃. From the developmental stage D to E , the number of rotifer ingested by fish decreased,and this tendency was most remarkable in group 18℃.
These results shows that it is more desirable to raise larval brown sole in Hokkaido region at 15 ℃ than at 18℃.
A-368 Hiroshi HOSHIKAWA
Laboratory observations of predation by three starfishes and one crab on hatchery reared juveniles of the abalone, Haliotis discus hannai
Predatory ability (PA) of three starfishes, Lysastrosoma anthosticta, Aphelasterias japonica, Asterina pectinifera and one crab, Pugettia quadridens, on the juveniles of abalone, Haliotis disucus hannai, produced artificially at the Hokkaido Institute of Mariculture, was surveyed in tanks from 1989 to 1990. The number of predated juveniles/predator/day was used as a parameter of PA to compare the seasonal and inter-specific variation. PA was high in L. anthosticta and A. japonica, followed by P. quadridens and A. pectinifera. Although no seasonal change of PA in A. japonica was observed, PA of L. anthostictha was low in winter and summer. The value of P. guadridens was low during the winter due to the low activity of the crab. The PA of A. pectinifera increased in winter season, because of the low activity of juvenile abalone. L. anthosticta and A. japonica predated large juveniles over 40mm in shell length. In contrast, A. pectinifera and P. quadridens fed on relatively small juveniles.
Predatory ability (PA) of three starfishes, Lysastrosoma anthosticta, Aphelasterias japonica, Asterina pectinifera and one crab, Pugettia quadridens, on the juveniles of abalone, Haliotis disucus hannai, produced artificially at the Hokkaido Institute of Mariculture, was surveyed in tanks from 1989 to 1990. The number of predated juveniles/predator/day was used as a parameter of PA to compare the seasonal and inter-specific variation. PA was high in L. anthosticta and A. japonica, followed by P. quadridens and A. pectinifera. Although no seasonal change of PA in A. japonica was observed, PA of L. anthostictha was low in winter and summer. The value of P. guadridens was low during the winter due to the low activity of the crab. The PA of A. pectinifera increased in winter season, because of the low activity of juvenile abalone. L. anthosticta and A. japonica predated large juveniles over 40mm in shell length. In contrast, A. pectinifera and P. quadridens fed on relatively small juveniles.
A-369 Shin-ichi NABATA, Akio TAKIYA and Masahide TADA
On the decreased production of natural kelp, Laminaria ochotensis in Rishiri Island, northern Hokkaido
An investigation on the fishing ground area and standing crop of the kelp， Laminaria ochotensis was carried out in Rishiri Island in July, 1998. Though the coralline flats have been in a trend of recovery, their area was about 133 ha.
The density of the kelp in Rishiri-cho Misaki was 111 individual/㎡for 1st-year fronds , 0.5 individual/㎡for 2nd-year fronds. The yearly production of natural kelp of Rishiri Island has decreased to 12% in comparison with the level of production in the 1940's. A correlation of the production of the kelp with the sea surface temperature was examined.
A significant negative relationship was found between the yearly production and the mean temperature in January, February, March of the previous year. It is concluded that the main factor leading to the decrease in kelp production was the increase in the relative amount of the Tsushima Warm Current. As a result, it is thought that the coastal water temperature in the winter period had increased, and grazing pressure by herbivorous animals on young sporophytes had increased.
An investigation on the fishing ground area and standing crop of the kelp， Laminaria ochotensis was carried out in Rishiri Island in July, 1998. Though the coralline flats have been in a trend of recovery, their area was about 133 ha.
The density of the kelp in Rishiri-cho Misaki was 111 individual/㎡for 1st-year fronds , 0.5 individual/㎡for 2nd-year fronds. The yearly production of natural kelp of Rishiri Island has decreased to 12% in comparison with the level of production in the 1940's. A correlation of the production of the kelp with the sea surface temperature was examined.
A significant negative relationship was found between the yearly production and the mean temperature in January, February, March of the previous year. It is concluded that the main factor leading to the decrease in kelp production was the increase in the relative amount of the Tsushima Warm Current. As a result, it is thought that the coastal water temperature in the winter period had increased, and grazing pressure by herbivorous animals on young sporophytes had increased.