Abstract: The capacity catch of two fleet segments - offshore trawlers and inshore purse seiners - are estimated through consideration of the maximum observed level of catch over a given time period. The results suggest that between 80-90 percent of the current catches could still be achieved following a one third reduction in fishing capacity. The optimal level of fishing capacity reduction, however, was not estimated.
The relationship among the catch (C), stock (N), fishing gear efficiency (q) and effort (E) is generalised in an equation as C = q · E · N. These factors, q, E, N are not constant from operation to operation in actual fisheries. The q and E are affected by various parameters, such as engine horsepower, gear size, the number and skill of crewmembers and others. These parameters are also generally combined to give a measure of fishing mortality, given by Fc = q · E, where the maximum of Fc is also an indicator of fishing capacity, and can therefore be used to derive a capacity index. In this paper, an analysis of the statistical distribution of catches per haul in trawl and purse seine fisheries in Japan is presented. The Fc was investigated and the relationship between reduction of the fishing capacity of a boat and the expected amounts of production was simulated.
Five fishing boats operating in the Northern offshore trawl and one coastal purse seine fleet were examined. The Northern offshore trawl is a medium-sized sector and the trawlers operate in an area of North East off Hokkaido (Northern Island). They produce a total catch of 4,000-5,000 tonnes a boat per year. The offshore trawlers included in the analysis were very similar technically. They were all equipped with fishing gear of similar size, and used a 1,800 SP engine and a 124 GT hull. The coastal purse seine fleet that was examined operates in the Seto inland sea in Western Japan. It consists of a pair of seiners (680 PS) and two carrier boats.
The catch data including the operation information were collected for a 3-year period (from 1989 to 1992) for the trawlers, and for a 1-year period (1997) for the purse seine fleet. Each value of catch per haul was transformed into a logarithm and its frequency distributions were analyzed. The frequency distribution curves were obtained for all the boats and main fish species. The method allowed approximation of the distribution with normal distribution curves, on which the relationship between reduction of the fishing capacity and the amounts of consequent catch was simulated.
No distinctive difference was found in fishing trend over the three years and between the five trawlers. The data for each species for all the boats and years were, therefore, amalgamated for the analysis of the species composition (Table 1). Walleye Pollock comprised of as much as 79 percent of all the catch in weight. Other fishes, such as Atka fish, flat fishes, red fishes and pacific cod were marginal in quantity. The total amounts of catch by the respective five fishing boats during the three years did not differ largely from each other, or ranged from 12 000 to 14 000 tonnes. The total numbers of hauls by the five boats for the three years ranged from 1 500 to 2 000, with an average of 1 778. The difference between the boats was also extremely small. The catch of walleye pollock by each boat was around 10 000 tonnes, which comprised a major proportion of the total quantity of catch. It was assumed, therefore, that the detailed analysis of walleye pollock should be representative of all catches.
Table 1. The proportion of the fish species captured by five trawlers in three yearsa
|
Fish species |
Scientific name |
Proportion |
|
Walleye Pollock |
Theragra chalcogramma |
79 % |
|
Threadfin hakeling |
Podnema longipes |
8 % |
|
Pacific cod |
Gadus macrocephalus |
1 % |
|
Atka fish, flat fish and red fish |
|
1 % |
|
Others |
|
11 % |
a). The total catch amount was 64 845 tonnes from 8 888 times of haul.
The catch per haul over the three years was analysed for each fishing boat. The distributions were found to be closely similar to normal distribution curves (Figure 1). The difference among the fishing boats was extremely small. This implies that the fishing activity undertaken by the studied boats is nearly the same. The catches per haul by each boat when targeting walleye pollock ranged mainly from 0.6 to 20 tonnes. They were highly concentrated, where as many as 800 times of operations by each boat fall around the centre of the distribution and only 50 times of operations distributed peripherally in the above range.
Figure 1. Distribution of catch per haul by offshore trawlers
On the basis of little difference in average catch among the boats, catches per haul from all the fishing boats for the three years were amalgamated and examined. For this purposes of the analysis, data of which the catches of walleye pollock were less than 100 kg were excluded, on the basis that they were most likely by-catch associated with the targeted catch of other species. The average catch per haul was 5.0 tonnes, and the standard deviation was as small as 2.6 kg. The 95 percent confidence interval for the average catch per haul was from 4.9 tonnes to 5.1 tonnes. It was concluded that the average catch per haul is some five tonnes when targeting walleye pollock and 3.3 tonnes in the case of operations of no particular targets in the Northern offshore trawl fishery operating in the area of North East off Hokkaido.
The largest quantities of catch per haul were, however, around 50 to 60 tonnes for all the boats in the three-year record. Those occurred only once or twice each year. This implies that the fishing capacity of the vessels is around 50 to 60 tonnes all the time, although the chance of fully utilizing such a large fishing capacity was five percent or less. According to a simulation, if fishers reduce their fishing capacity to 8.3 tonnes, as the upper limit of catch quantity per haul, they would maintain the catch equivalent to 70 percent of the current production and the loss in comparison to the current production could be 30 percent. If they limit the fishing capacity to 11.5 tonnes, they would maintain the level at 80 percent of the current production, while capacity of 17.4 tonnes would enable 90 percent of the current production. However, if they want to maintain the 95 percent of the current catch, a fishing capacity of 24.5 tonnes would be required.
The target species for the examined coastal purse seine were jack mackerel, mackerel and sardine. The nets used during the studied period were some 672m in cork line length and 195m deep. The nets for respective target species were similar in size, despite the deference in mesh size. The frequency distribution of catch per haul is shown in Figure 2. For the purposes of the analysis, empty hauls (which occurred in ten percent of the cases when targeting jack mackerel and mackerel) were excluded from the analysis. The catches per haul for jack mackerel and mackerel ranged from 0.06 tonnes to 10.4 tonnes, and those of sardine from 0.8 tonnes to 130 tonnes. The largest amount of catch per haul was 173 times greater than the smallest for jack mackerel and mackerel and 163 times greater, for sardine. The largest amount of catch per haul occurred only once or twice a year. The statistical distribution patterns of the catch per haul for the three fishes were similar to those for walleye pollock from the trawlers.
Figure 2. Distribution of catch per haul by coastal purse seine
The fish stock exploited by the studied fishing sectors fluctuates from year to year, as does the total allowable catch (TAC). Control of fishing capacity is necessary under such a circumstance. In other words, fishing capacity must be adjusted frequently. It is, however, not realistic to change each year the number of boats or the ability of a boat such as the size, engine horsepower, fishing devices onboard every year. These factors should be changed from the long-term viewpoint over 20 to 30 years. The realistic fishing control corresponding to the fluctuation of fish stock should be applied to fishing gear and operational methods.
The actual catch per haul with the same fishing capacity differs by a factor of 100 between the smallest and the largest. In both the studied fishing sectors, the largest catch occurred only once or twice a year. This implies that, theoretically, 80-90 percent of the current catch could still be achieved even if fishers reduce their fishing capacity by one third from the present condition. However, the appropriate rate of reduction of the fishing capacity has not been clarified yet, because there are no actual data under the condition of reduced fishing capacity. The reduction rate should be decided on the basis of a survey of current fisheries and trial operations prudently.
This study suggests that it is possible to reduce the fishing capacity in current capture fisheries. The method used demonstrates that it is possible to assess fishing capacity from catch records even if there is no detailed survey on fishing gear and fishing boat. The present fishing gear do not have the function or structure to change their fishing capacity, however, it must be possible to design such gear by a systematic approach from the beginning of the development.
