Kunio Shirakihara

Finless porpoise abundance in Omura Bay, Japan : estimation from aerial sighting surveys

We used aerial surveys and line-transect methods to estimate the abundance of the finless porpoise (Neophocaena phocaenoides) in Omura Bay, Japan. We conducted 4 surveys once each season from August 1993 to May 1994. We surveyed 621.5 km and sighted 54 porpoise groups (87 animals). In the spring survey, we detected porpoises offshore as well as inshore, whereas porpoises were mostly inshore in all other seasons. The abundance of porpoises was estimated to be 187 animals (0.6 individuals/km 2 , CV = 20%).

A population size estimate of the finless porpoise,Neophocaena phocaenoides, from aerial sighting surveys in Ariake Sound and Tachibana Bay, Japan

We estimated the population size of the finless porpoise (Neophocaena phocaenoides) in Ariake Sound and Tachibana Bay of western Kyushu, southwestern Japan, from aerial sighting surveys using line transect methods. All 12 surveys were conducted from May 1993 to May 1994 (8 in Ariake Sound and 4 in Tachibana Bay). In addition to these, 14 surveys were also carried out to obtain information on porpoise occurrence in Tachibana Bay (5 surveys) and in neighboring Sumo Nada (5) and Yatsushiro Sound (4). In Ariake Sound, 225 porpoise groups (369 animals) were detected during all flights totalling 1,694.4 km. In Tachibana Bay, a total of 997.8 km was surveyed and 55 groups (290 animals) were sighted. However, no sightings were recorded in Sumo Nada (distance searched = 148.7 km) and Yatsushiro Sound (208.4 km). In Ariake Sound, few sightings were recorded from waters shallower than 5 m in depth. In Tachibana Bay all animals were detected from waters of less than 50 m depth. The population size was estimated as 1,983 animals in Ariake Sound (95% CI = 1,382-2,847), 1,110 in Tachibana Bay (95% CI = 642-1,920), and 3,093 in the 2 waters (1.3 individuals/km2, 95% CI = 2,278-4,201).

FOOD HABITS OF FINLESS PORPOISES NEOPHOCAENA PHOCAENOIDES IN WESTERN KYUSHU, JAPAN

Abstract We examined the stomach contents of 87 finless porpoises (Neophocaena phocaenoides) in the Ariake Sound–Tachibana Bay area and Omura Bay in western Kyushu, Japan, between 1987 and 1992. Fish (Gobiidae and Atherinidae) were the most numerous and most frequently occurring prey in Omura Bay, whereas both cephalopods (Octopodidae, Sepiidae, Sepiolidae/Sepiidae, and Loliginidae) and fishes (Clupeidae, Engraulidae, and Sciaenidae) were equally important in Ariake Sound–Tachibana Bay. Species compositions in the commercial catch differed between the 2 waters, suggesting that differences in prey availability may explain the geographical variation in diet. Finless porpoises in Ariake Sound–Tachibana Bay showed ontogenetic and seasonal variations in diet. The mean length at weaning was estimated to be 101 cm, corresponding to approximately 6 months of age. Calves fed on small-sized demersal fish and cephalopods. The predominant prey species for sexually mature individuals (including lactating females) consisted of konoshiro gizzard shad (Konosirus punctatus) and cephalopods. Seasonal availability of these organisms may be related to births in the fall–winter season in Ariake Sound–Tachibana Bay. Size-dependent prey selectivity was equivocal. Day–night difference in foraging time was not indicated by the index of stomach fullness.

Two fish species competition model with nonlinear interactions and equilibrium catches

Two species competition model is built up by assuming the hypothetical second order interactions in order to consider effects of exploitation on two competing fish species with non-linear interactions. Most important characteristic of this model, compared withLotka-Volterra type linear competition model, is that this model can possess multiple stable equilibrium points. Therefore there is a possibility that two species keeping the equilibrium state at one stable equilibrium point will be attracted to the other stable equilibrium point after a heavy perturbation. In this model reversible change of the fishing pressure does not always results in that of the equilibrium catch. In this sence MSY concept for single species can not be extended to this model. If there are multiple stable equilibrium points, the change of the dominant fish species, catastrophic and irreversible change of each equilibrium catch may be observed when the perturbation by the exploitation is added. This phenomenon immediately reminds us of the change of the dominant fish species between Japanese common mackerel and Pacific saury in the northwest Pacific Ocean. In case of the management of two competing fish species with nonlinear interactions, the consideration on the balance between the fishing pressure for each species may be as important as the decision on the catch limit for each species. MSY level for each species based on the single-species theory could be quite erroneous.

A feedback management procedure based on controlling the size of marine protected areas

A feedback management procedure is proposed that is based on controlling the size of a Marine Protected Area (MPA): increasing the size of the MPA when the population is below a predetermined target level and reducing its size when it is above. The objective of fisheries management is to have the population size approximate the target level. Local stability was analyzed, to determine whether a population could remain close to the target level with small perturbations, using a generalized population dynamics model without including an age structure. The procedure was able to sustain the population at the target level if the target was set at a level that gives the maximum sustainable yield and if drastic changes in the MPA size were avoided. Global stability was analyzed numerically to determine whether a population would always approach the target level regardless of the present population size, using a specified model that allowed for temporal changes in the spatial distribution of a sedentary population. The procedure, which did not require direct control of catch quota or fishing effort, appeared to attain the objective successfully.

Stock size estimation of red sea bream in the Chikuzenkai from cohort analysis.

In the Chikuzenkai catch statistics by age and fishery were prepared on red sea bream and stock size was estimated using the cohort analysis in order to get fundamental information on the management.The ages in the catches ranged from 0 to 6 years old with composition extremely skewed to the younger ages less than 3 which were exploited mainly by one-boat gochi net (surrounding seine) and two-boat gochi net. The older fish were exploited by angling, longline and bottom gillnet.The number of fish recruited at July of age 0 (three months old) were estimated to be 22-34 millions from 1978 to 1982. The mean survival rates were 24% to age 1, 7% to age 2, 2% to age 3. Reduction of the high fishing pressure to the younger fish was suggested to increase the catches of the older and larger fish.