Based on catch and effort data analyses covering the period 1996–2002, time series of catch rates in the trawl fisheries in the South China Sea along the coasts of Bac Lieu and Ca Mau in South East Vietnam were estimated.
The indicators include catch rates for total shrimp catch, five major shrimp catch groups and 15 commercial shrimp groups estimated as quarterly averages for trawlers divided into five size groups.
The analyses indicated that despite a reduction in catch rates, shrimp is still the principal resource for the trawl fleets in South Vietnam. Seasonality in the shrimp catch rates that might reflect seasonality in shrimp recruitment was found, making this resource potentially suitable for a fishery management system based on closed seasons. Further, the data indicate that the major part of the catches are comprised of low value species belonging to the genera Parapenaeopsis; whereas the most valuable species, i.e. the Penaeus and Metapenaeus catch groups have been significantly depleted during the period investigated.
Based on the experiences from the present analysis, recommendations are presented with regard to adjustments of the enumerator data collection programme to fulfil the requirement of a dynamic fishery management system.
The resources of shrimp in Vietnam are characterised by a high number of species (about 225) belonging to 68 genera of 21 different families. The most important shrimp family for the commercial fishery is Penaeidae with 75 species and 3 genera.
The life cycles of the penaeid shrimps have several features in common. In the Lower Mekong Delta (LMD) region large juvenile and adult shrimps can be found close to the shore or inside the rivers and tidal channels. They are generalists and feed opportunistically on organisms living in the bottom sediments: i.e. bacteria, algae, meiofauna and small macrofauna.
The distribution of penaeid shrimps might be based on environmental factors such as sediment, salinity, transparency of the water and they may be divided into groups (e.g. estuarine, island/offshore, rock sediments/sponge groups) according to their adaptation at various life stages.
The nursery habitats for the majority of peneid shrimps are found in the estuarine environments. Therefore, these shrimps are entirely dependant on contact with the estuarine environment in order to complete their life cycle.
As the shrimps grow larger they tend to migrate from the shallow nursery areas to the deeper and more saline waters further offshore where spawning takes place.
The peneid shrimps are exploited by various types of fisheries at different stages of their life cycles. In the nursery areas, the large juveniles are exploited by coastal and mangrove river-canal bag net fisheries and by the mangrove shrimp pond culture industry. As the shrimps mature and migrate offshore, they are recruited to the offshore fisheries. The major part of the marine shrimp catches in Vietnam is taken by the otter trawl fleet whereas the contributions from push netters, trammel nets and other gear types are insignificant at the national level.
Fisheries in Vietnam are, in principle, open access fisheries where only a technical approval of a vessel is required to obtain a fishing licence. The shrimp fishery is regulated by means of gear restrictions (including a ban on using push nets and other destructive gears) and technical measures such as closed areas and minimum trawl mesh size. However, in many situations, enforcement is limited and vessels may fish without a licence and illegal gears (e.g. push nets) are still in use in many areas.
The principal shrimp fisheries in Vietnam are in the South China Sea by trawlers from Bac Lieu and Ca Mau provinces. The official catch statistics for these two provinces are indicated in Table 1. This information is collected by statistical officers in the communes interviewing a few fishers once or twice a year. From the district level the catch and effort information are reported to the district officers. After checking and aggregating the data at the district level the district officers forward the information to the statistical departments at the provincial level, where it is submitted to the Ministry of Fisheries in Hanoi. The official fishery statistics have a poor categorical resolution with catch data always being indicated as total weights of “fish,” only sometimes broken down by commercial categories or industrial products. For a thorough discussion on the official data collection programmes in Vietnam see Van Zwieten et al. (2001).
In Ca Mau and Bac Lieu about 1600 otter trawlers with engines of different capacities were licensed by the provinces in 2002 as indicated in Table 2. Trawlers from Bac Lieu are mainly fishing in the South China Sea whereas trawlers from Ca Mau fish in both the South China Sea and in the Gulf of Thailand as the province borders both areas.
This paper presents an evaluation of data from the otter trawl fishery in South East Vietnam exploiting the peneid shrimps in the South China Sea. The analysis is based on enumerator data covering the period 1996 to 2002 from Ganh Hao landing site in Bac Lieu province. Data from this site are assumed representative for all shrimp trawlers from Bac Lieu province and for the trawlers from Ca Mau province fishing in the South China Sea.
Since 1996, DANIDA1 has supported the establishment of a fisheries data collection programme based on statistical samplings of the principal fleets in the principal landing places in all of the 28 coastal provinces in Vietnam. The data collection programme is based on local enumerators interviewing crew members of vessels as they return from a fishing trip. Before selling the catch (most often to middlemen) the fishers sort it into catch groups by genera. If certain species in a catch group appear in sufficient numbers they are sold in separate monospecific commercial groups obtaining higher prices. The vessels to be interviewed are selected at random and each interview relates to catch weight (by commercial group), effort (duration of trips), costs and earnings of the most recent trip only. As a general rule, each month the enumerators carry out 20 interviews for each fleet included in the sampling programme but the actual number may be less due to various sampling difficulties.
The present analysis is based on interviews relating to more than 26.000 otter trawl landings in Ganh Hao landing place in Bac Lieu province during the period 1996 to 2002. The location of the landing site is indicated in Figure 1.
To focus the analysis on the development of the shrimp resource rather than on fleet performances, only data from fishing trips with shrimps in the catches were included in the analysis. Furthermore, in the present study the indicators (catch rates of total shrimp catch, shrimp catch groups and commercial groups) were estimated by fleet as quarterly averages. Only estimates that could be based on 5 samples or more in a month were included in the present analysis.
Five otter trawler fleets are defined on basis of vessel size: less than 20HP (HP < 20), between 20 and 45 HP (HP20-45), between 46 and 89 (HP46-89), between 90 and 140HP (HP90-140) and larger than 140HP (HP > 140).
As indicated in Table 3 there are five different shrimp catch groups (based on genera) made up by fifteen commercial groups of which ten are monospecific. Within each catch group there is one commercial group with mixed species composition, comprising those shrimps that for one reason or another are not sorted out into monospecific categories.
Shrimp catch rates
Considering the total shrimp catches, Figure 2 indicates that the shrimp catch rates are related to vessel size and that catch rates of all but the smallest trawlers have declined over the time period. The smallest vessels (HP < 20) had shrimp catch rates about 2 kg per hour throughout the whole time span. The decline in catch rates for the larger trawlers (HP > 140) was about 40% (from about 10 to 6 kg per hour) whereas the medium sized trawlers (HP 20–45 and HP 46–89) both declined about 30%. However for all fleets, seasonal and inter annual fluctuations have been significant, making identification of trends of the short time series difficult.
Although presented as quarterly averages, the catch rate time series show indications of seasonal variation. The catch rates tend to increase by the end of a calendar year (up to 14 kg per hour for the larger vessels) and to decrease during the first few months of the following year. However, after the second half of 2001, major peaks have been absent and catches fluctuated about 2 kg hour− 1 for small vessels (HP < 20) and between 2 and 7 kg hour− 1 for the other fleets.
When measuring shrimp catch rates against total catch rates, it was found that although the smallest vessels have the lowest catch rates (Figure 2), they have the highest proportion (50–90%) of shrimps in the catches throughout the time series as indicated in Figure 3. The average proportion of shrimps in the catches of the larger vessels was about between 40 and 65% before 1998. After 1998 the shrimp contributions fluctuated between 20 and 50% apart from the seasonal peak in 2000/2001 when the proportions of shrimps rose to about 60%.
Measuring the contribution of Cat prawns to the total shrimp catch rates indicated that for all fleets Cat prawns were by far the most abundant in the catches most of the time until end of 2001 (Figure 4). During the peak seasons between 60 and 90% of the shrimps belonged to the Cat prawn catch group. The peak seasons were separated from each other by one or two quarters with low catch rates.
During the peak seasons in 1998 and in 2001 the average Cat prawn catch rates were about 4–5 kg hour− 1 and it was up to 8 kg hour− 1 during the peak in 1999/2000 (Figure 5). After the decline in catch rates by end of 2001 they remained at a very low level throughout 2002.
As also indicated in Figure 5, until 2002 only a minor part of the Cat prawns were sold in the monospecific category (Parapenaeopsis sculptilis), the rest being sold as the unsorted mixed Cat prawn catch group of less value. In 2002 when the Cat prawn catch rates remained very low, almost all were sold in the P. sculptilis monospecific group.
Low value shrimps
As indicated in Figure 6, the contribution of low value shrimps to the total shrimp catch rates fluctuated in counter phase with those of the Cat prawns (see Figure 4); i.e. the low value shrimps comprised the major part of the shrimp catches (between 40 and 90%) in times of low Cat prawn contributions. During 2001 the small trawlers (HP < 20) had a significantly smaller proportion of low value shrimps than the other fleets; but during whole of 2002 (when the Cat prawn catch rates were low) the contribution of low value shrimps remained high for all trawler fleets.
In average, low value shrimp peaks were between 4 and 6 kg hour− 1 until the second quarter of 1999 (Figure 7). In both the second half of 1999 and in 2002 the peak catch rates were reaching 8kg hour− 1 and in last quarter of 2000 and first quarter of 2001, when the spear shrimp commercial group appeared for the first time, the catch rates reached 10 kg hour− 1. In 1998 and 1999 the Trachypenaeus commercial group was dominant and in 2002, Dog shrimps were also sold in a monospecific group.
As indicated in Figure 8, the average contribution of Pink prawns to the total shrimp catch rates declined from a level fluctuating around 10% (between 3 and 18%) in the period 1996–1999 to a level fluctuating around 6% (between 1 and 18%) in 2000.
From 1996 to 1999 the quarterly catch rates of Pink prawns fluctuated between 0.6 to 1.8 kg hour− 1, whereas they fluctuated between 0.3 and 0.6 kg hour− 1 from 2000 onwards (Figure 9).
It appears that from 1996 to the third quarter in 1997 and again from the third quarter of 2000 onwards, Pink prawns were sold almost entirely as Greasybacks (Figure 9). In between, i.e. from the fourth quarter of 1997 to middle of 2000, they were rarely sorted out into monospecific commercial categories.
The contribution of Yellow shrimps to the total shrimp catches have declined for all fleets (Figure 10). The Yellow shrimps were most dominant in the catches of the small trawlers (HP < 20). Until the end of 2001 the contribution of Yellow shrimps to the total shrimp catches of this fleet fluctuated around 15% (between 7 and 35%). In 2002 the contribution fluctuated between 5 and 10% only and the otherwise rather frequent spikes were entirely absent in 2002. As also indicated in the Figure 10 the abundance of Yellow shrimps declined from about 10 to 5% in catches of medium sized trawlers (HP 20–45 and HP 46–89) and almost entirely disappeared from the catches of the large trawlers (HP > 140).
The average catch rates of Yellow shrimps declined from about 0.8 kg hour− 1 in 1996/1997 to 0.15 in the first quarter of 2001 (Figure 11). In 2002 the catch rates fluctuated between 0.17 and 0.25 kg hour− 1.
Since 1998 the Yellow shrimp catches were comprised almost entirely of the M. tenuipes (Figure 11) whereas catches of M. brevicornis were reported by the enumerators in first quarter of 1999 and fourth quarter of 2000 only.
Apart from the small trawlers (HP < 20) occasionally having up to 3.5%, White prawns comprised less than 1.5% of the shrimp catches of all other fleets (Figure 12). From 1996 to 1998 the White prawn to total shrimp catches fluctuated around 1%. After 2000 the contribution of White prawn fluctuated round 0.5% for the medium sized vessels (HP 20–45) and at even lower levels for the larger fleets.
From fourth quarter 1996 to fourth quarter in 2000 the catch rates of White prawn fluctuated between 0.1 and 0.02 kg hour− 1 (Figure 13). In 2001 and 2002 catch rates fluctuated between 0.006 and 0.02 kg hour− 1 apart from the fourth quarter in 2002 having catch rates of 0.08 kg hour− 1.
The White prawn category consisted in 1996 and 1997 primarily of Banana prawns (Figure 13). Since the beginning of 1998, most White prawn were not sorted into any monospecific categories but rather sold as the White prawn mixed commercial group. Catch rates of Tiger shrimps were high in 1998 and 1999, but otherwise occurred rarely in the catches.
Using the time series of catch rates as indicators of abundance, the enumerator data from Ganh Hao landing place suggests that there was a significant decline in shrimp stock biomass from 1997 to 2002. Local enumerators and the staff of the Fisheries Department in Ca Mau made it known that in 2001 and 2002 several trawlers changed gear and shifted target species from shrimp to small pelagic fish, due to low profitability. As statistical data on total effort and effort allocation was not available for the present analysis, trends in realised effort in the shrimp fisheries could not be estimated. Using data from the trips with shrimps in the catches only is believed to have excluded most of the trips where effort was directed towards other species, focusing the analysis on the shrimp resource. Although the total effort measured as the annual number of trips, or the total number of fishing days targeting shrimps remains uncertain, it is assumed to have declined in 2001 and 2002. Therefore, the decline in the shrimp stocks might have been more severe than indicated by the decline in the catch rate time series.
The cyclic fluctuations in Cat prawn catch rates may suggest seasonality in their recruitment to the trawl fishery reflecting their migratory life cycle. Consequently, the absence of a catch rate peak in 2002 could be a warning of recruitment failure in the Cat prawn stock. As the enumerator data does not include information on the average size of the shrimps in the catches, it was not possible to verify whether there were seasonal changes in size distribution as would be the case if juveniles, migrating from the coastal nursery areas to offshore spawning grounds, dominated the catches at the beginning of the season. However, this hypothesis is supported by anecdotic information from the enumerators in Ca Mau, confirming that during the periods with increasing catch rates, the average size is much smaller than later in the season.
Comparing the Cat prawn and the low value shrimp contributions (Figures 4 and 6) it appears that they fluctuated in counter phase with each other. From the catch and effort data made available for the present study it was not possible to analyse this relationship in further detail. However, according to the local enumerators, the shrimpers compensate for decreasing catch rates by moving to secondary fishing grounds with less profitable species composition to maintain their income.
In a rational fishery, the fishing strategy is affected by resources, fishing costs and market conditions. With regard to the shrimp fishery in South Vietnam as reflected in the enumerator data, market prices fall in tree distinct categories as follows (2002 prices): white shrimps (110.000 dong kg− 1), Pink prawn (45.000 dong kg1), and Cat prawns, low value shrimps and yellow shrimps (20.000 dong kg− 1).
The decline in catch rates of White and Pink prawns might have had a significant negative impact on the profitability of the shrimp fishery. This (in combination with the decline in international shrimp prices since beginning of 2000 and increased fuel costs) might have lead to changes in fishing strategies in 2001 and 2002, as the anecdotic information from the enumerators and Fisheries Department in Ca Mau also suggests. Furthermore, as the most valuable stocks are being decimated, more effort is likely to be directed towards the lower value species. Therefore, the recruitment failure in the Cat prawn stock in 2002 as indicated by the enumerator data may very well be the result of increasing fishing pressure leading to recruitment overfishing.
It is difficult to use the enumerator data to discuss changes in the species composition, as each of the catch groups include a commercial group of mixed species (Pink prawn, White prawn, Cat prawn) with unknown species composition. The monospecific commercial groups are used only when there are sufficient catches of a certain species. Species with low catches are grouped together in the mixed species commercial groups. Without information from additional surveys and sampling programmes about the species composition within the mixed groups, it is difficult to discuss consistently whether a sudden absence of monospecific commercial groups in the enumerator data reflects low catches or just changing market conditions.
In order to establish catch rate time series of indicator species that are more robust and informative about the state of the shrimp resources, additional data should be collected on a routine basis to identify the proportion of key shrimp species (e.g. those having their own monospecific commercial group) in the mixed species commercial groups. Furthermore, the enumerator data collection programme should be modified to also measure the number of shrimps in a sample of known weight for each interview. This would be an easy and cost effective way to obtain valuable information on average size of the shrimps that could be supportive of a fishery management system based on closed seasons.
The enumerator database comprises detailed information about the performance of the fishing fleets in Vietnam and could prove a valuable tool in establishing time series of fisheries indicators supporting fisheries management. However, taking into consideration the complex life cycle of the peneid shrimps, it is suggested that the enumerator data collection programme should be extended to include fisheries at all life stages of the shrimp, including data from the mangrove and river-canal bag net and the mangrove pond fisheries. Catch rate indicators based on data covering the complete life cycle the indicators would be more informative and supportive for fisheries management.
Danish International Development Agency.