E/19
STOCK ASSESSMENT IN THE SHRIMP (Heterocarpus reedi) FISHERY OF CHILE

by

D.A. HANCOCK ¹
FAO/UNSF Fisheries Officer (Biology)

and

G. HENRIQUEZ
Instituto de Fomento Pesquero
Santiago, Chile

Abstract

A review has been made of the available information relative to a stock assessment of the shrimp fishery of Chile. Exploratory surveys have shown that some lightly exploited stocks of Heterocarpus reedi still exist. A careful analysis of existing data on catch and effort shows the catch per trip to be variable, but there is an indication of fewer, but longer trips each month. The average catch per trip increased between 1965 and 1966. The analysis is complicated by the heterogeneity of the fleet, and the use of the same vessels for other species, primarily langostino, and so far there has been no reliable information on the length of trips. There was no detectable difference between size compositions in 1958–59 and 1965–66. Male shrimps generally constituted well under half of the catch, and the majority of females were ovigerous during the winter months. The highest recorded annual catch of shrimps was for 1966 and the evidence suggests that the landings may continue to increase.

¹ Permanent address: Fisheries Laboratory, Burnham-on-Crouch, Essex, England

PECHERIES CHILIENNES DE LA CREVETTE Heterocarpus reedi: EVALUATION DES STOCKS

Résumé

On a passé en revue les renseignements disponibles permettant de procéder à une évaluation des stocks des pêcheries chiliennes de crevettes. Des campagnes de prospection ont révélé qui'il existe encore des stocks de Heterocarpus reedi peu exploités. Après examen attentif des données existantes sur les captures et l'effort de pêche, on constate une variabilité de la prise par sortie, mais il semble que les voyages mensuels se fassent moins nombreux et plus longs. La prise moyenne par sortie a augmenté de 1965 à 1966. L'étude est compliquée par l'hétérogénéité de la flotte, ainsi que par l'utilisation des mêmes navires pour la pêche d'autres espèces, notamment le “langostino”; en outre, on ne dispose pas jusqu'ici de renseignements sûrs concernant la durée des voyages. On n'a pas décelé de différences dans la composition par tailles entre 1958–59 et 1965–66. Dans l'ensemble la proportion des mâles dans les captures est loin d'atteindre 50 pour cent, et la plupart des femelles sont ovigères durant les mois d'hiver. C'est en 1966 qu'a été enregistrée la plus forte prise annuelle, et tout porte à croire que les mises à terre pourront continuer á progresser.

EVALUACION DE POBLACIONES EN LA PESQUERIA DE CAMARONES (Heterocarpus reedi) DE CHILE

Extracto

Se ha realizado un estudio de la información disponible relativa a la evaluación de las poblaciones de la pesquería camaronera chilena. Los estudios exploratorios han mostrado que todavía existen algunas poblaciones poco explotadas de Heterocarpus reedi. Un análisis cuidadoso de los datos existentes sobre captura y esfuerzo, muestra que la captura por viaje es muy variable, pero existen indicaciones de que se realizaron menos viajes, pero más largos, cada mes. La captura media por viaje aumentó entre 1965 y 1966. Complica el análisis la heterogeneidad de la flota y el empleo de los mismos barcos para otras especies, principalmente langostinos, y hasta ahora no se ha dispuesto de información fidedigna acerca de la duración de los viajes. No hubo diferencia apreciable entre las composiciones por tamaño en 1958–59 y 1965–66. Los camarones macho constituyen en general bastante menos de la mitad de la captura, siendo ovígeras la mayoría de las hembras durante los meses de invierno. La mayor captura anual de camarones registrada correspondió a 1966, y la experiencia indica que es posible que continúen aumentando las cantidades que se desembaroan.

1 INTRODUCTION

In 1954, Lindner (1957) examined the shrimp fishery of Chile and concluded that production was very small, amounting to only about 100 t, including heads. By 1966, the shrimp fishery had increased to more than 10,000 t. In addition, landings of “langostino” had increased from 2,600 t in 1954 to over 11,000 t in 1966 (Fig. 2).

The species of shrimp of potential importance in Chilean waters (Mistakidis and Henríquez, 1966) are three:

1. Rhynchocinetes typus H. Milne Edwards, the “camarón de playa”. A small to medium sized shrimp of the family Rhynchocinetidae which is fished close inshore with small traps, and, before 1953, comprised most of the landings of shrimps.

2. Heterocarpus reedi Bahamonde, the “camarón nailon”. A pandalid shrimp which accounts for about 95 percent of present landings. Mistakidis and Henríquez (1966) recorded that the deep-sea fishery for this species commenced in 1953, when boats trawling for hake in depths of 180 m encountered concentrations first of langostino, and later of H. reedi, which is now fished in depths of more than 200 m.

3. Hymenopenaeus diomedeae (Faxon), the “gamba”, is a penaeid shrimp, larger than the other two species, which is trawled in depths beyond the 150 fm (275 m) line. Although there have been reputed catches approaching 1 t per h, there have been no commercial developments involving this species.

Langostino species must also be mentioned since they are often caught with shrimps, thus creating problems in the analysis of catch and effort data. In Chile the name is used for species of Galatheidae, Cervimunida johni Porter, the “langostino amarillo” and Pleuroncodes monodon (H. Milne Edwards), the “langostino colorado”, the first of which is the more important. Langostino are normally fished in depths from 100 to 250 m.

Biological and economic studies on the shrimp and langostino fisheries commenced at the Instituto de Fomento Pesquero (IFOP) in January 1965, and reports have been presented on exploratory surveys (Mistakidis and Henríquez, 1966; and unpublished work) on stock assessment (Saetersdal, Henríquez and Sanhueza, MS), and on industrial aspects of the fishery, processing and marketing (López-Capont et al., 1965). Before 1965, information on Chilean shrimps was limited to the biological and taxonomic publications of Bahamonde (1955; 1958; 1963) and of Bahamonde and López (1960), a general account of the shrimp fishery by Lindner (1957) and statistics of landings collected by the Department of Fish and Game of the Ministry of Agriculture.

The aim of the present paper is to review the existing information, and to discuss ways of improving the collection of data necessary for a stock assessment of the shrimp fishery.

2 THE FISHERY

The most important landing ports, for both shrimps and langostino, are Valparaíso, San Antonio, Quintero and Coquimbo (Figs. 1 and 3). At Valparaíso, which is responsible for more than half of the country's landings, there are five companies which in 1966 operated a fleet of 16 boats (Table I), of extremely diverse size, age and construction, working principally for shrimp and/or langostino. There were a further 20 boats working from the remaining ports; these have almost all been modified for trawling from some other purpose, and range from 38 ton wooden purse seiners to a 280 ton steel-hulled whaler (Table I). At Quintero there is a small fleet of German trawlers.

The species receiving the major effort may depend on a variety of factors including its availability, the demand (especially for shrimps) and the incidence of moulting (langostino). The by-catch usually includes hake (merluza) and occasionally the same boats will concentrate on trawling for this species. This was one reason for the poor shrimp landings during 1963 (Fig. 2).

Fig. 1 Distribution and main fishing grounds for Heterocarpus reedi.

Shrimp and langostino may be taken together in the same hauls, but, since shrimps are usually found in deeper waters than langostino, fishing is likely to be directed towards one species or the other (Figs. 13 and 14). The best grounds for shrimps and langostino are from Coquimbo to Valparaíso and off San Antonio and Constitución (Fig. 1), roughly between 29 and 35°S. In these areas boats may expect to take up to 1,600 kg/h of shrimp or 2,000 kg/h of langostino. Mistakidis and Henríquez (1966), aboard a commercial boat in March 1965, observed smaller catches at night, and suggested this was because the shrimps were off the bottom at that time.

Shrimps are normally taken on a bottom of muddy sand, clay or compacted clay, while langostino are generally confined to areas of sand and muddy sand with less clay. The gear for both species therefore does not normally need to be designed for very rough bottoms.

The normal gear is an otter trawl, modified from a German fish trawl, with heavy doors, each weighing about 350 kg, depending on the size of vessel and made from iron or iron and wood. A typical net is made of nylon or polypropylene, with a 95 ft (29 m) footrope, 81 ft (25 m) headline, and a stretched mesh of about 4 cm for shrimps and 5 cm for langostino. The net for one species is frequently not changed when fishing for the other, and there are no regulations covering the mesh of nets. Regulations do exist specifying a minimum size (2.5 cm carapace length), a close season (1 September - 31 December) and the prohibition of landing of berried females, but in no case is the species of marine shrimp specified.

The majority of boats are at sea for less than 24 h, and none are away from port for more than 3 days. This is due partly to the limited range of the smaller boats and partly to the lack of refrigeration facilities. With present conditions of storage on ice, spoilage will occur rapidly after 3 days. Only a small proportion of landed shrimps and langostino are consumed fresh; the remainder are cooked on shore at the processing plants, either by boiling or under steam, and usually peeled by hand. Hand peeling is often stipulated by overseas buyers. The average yield of cooked flesh per total weight landed is about 17–18 percent for shrimps compared with 8–9 percent for langostino.

A large fraction of the cooked shrimp is frozen while the remainder (less than 10 percent) is canned. A very high proportion of the total landings of both species is exported, and the value of exports has risen steadily during the past few years. By 1965, the total export of shrimps, mostly frozen, many of them for the United Kingdom, reached a value of US $1.25 million. Langostino exports reached a total value of $1.75 million in 1965, mostly frozen for the United States.

During 1966, more processing plant has been added, some of it with modern equipment, and total landings of shrimps showed a marked increase in that year (Fig. 2). The value of exports for 1966 totalled US $4.6 million, which included US $2.6 million of shrimps.

3 DISTRIBUTION AND EXPLORATORY FISHING

Heterocarpus reedi has so far not been identified anywhere farther north than Taltal nor farther south than Puerto Saavedra (Fig. 1), but information is still lacking on its complete distribution. It is similar in appearance to H. hostilis Faxon, which was taken between Panamá and the Galápagos Islands, but it is more like H. dorsalis Bate which is not found in the eastern Pacific. The three species differ in a number of characteristics (Bahamonde, 1955).

Neither shrimps nor langostinos have been encountered in commercial densities in Chile north of Coquimbo, but to the south good catches have been reported by boats trawling for hake off Talcahuano (Fig. 1). In view of the increasing demand for frozen shrimp and langostino, the Chilean Development Corporation (CORFO) arranged the first exploratory survey of this area in April 1964. The relatively few hauls which yielded good catches contained up to 1,300 kg/h of shrimps and 1,700 kg/h of langostino (Mistakidis and Henríquez, 1966) and were sufficiently encouraging for more observations to be made in the area.

Fig. 2 Annual landings of shrimps and langostino; data supplied by the Department of Fish and Game (1952 to 1966) and IFOP (1965 to 1966).

Fig. 3 Annual shrimp landings by ports, 1960 to 1966, data from the Department of Fish and Game (1960 to 1964) and IFOP (1965 to 1966).

TABLE I

The fishing fleet at Valparaiso 1960–1966, showing the number of calendar months during which boats fished for shrimps or langostino and details of their construction

TABLE II

Average numbers of trips per month from Valparaiso, obtained by comparing the same boats (A) in pairs of years from 1961 to 1966 (this procedure was necessary because the same boats were not represented throughout the period) and (B) boats which fished in 1964, 1965 and 1966

In October and November 1965, Mistakidis and Henríquez (1966) made a more detailed trawl survey of the area between 35° and 39°S (Fig. 1). They concluded that commercial concentrations of shrimps and langostino occurred north of latitude 37° (near Talcahuano) but not farther south. A re-examination of the area at the end of March 1966 confirmed this pattern of distribution, but gave somewhat lower catches of shrimps. The November maximum and average catches were about 1,740 kg and 530 kg per h respectively, compared with around half these quantities in March. The authors suggested that the difference may have resulted from a horizontal migration at the approach of winter.

The area between 35° and 37°S, which had been shown to contain commercial densities, was resurveyed in November 1966 in conjunction with a foreign company, but this time using different gear supplied by the company. As in the previous November, maximum catches of shrimp exceeding 1,700 kg/h were taken, with an average of 750 kg/h (Fig. 13). The company made an immediate application for permission to establish plant at Talcahuano, where, to date, shrimp and langostino have not been processed.

A short survey off Taltal (Lat. 25°S) in April 1966 failed to locate more than a few specimens of shrimp and langostino.

4 CATCH AND EFFORT

The published statistics of the Department of Fish and Game give information only on monthly and annual landings by ports, but details of the boats and the number of trips each month have also been supplied by them privately. Fig. 2 and Table III show the annual landings of shrimps and langostino, both of which have been increasing fairly steadily. The weight of the shrimp catch has always been much smaller than that of langostino. Mistakidis and Henriquez (1966) suggested that this may have been due to lack of adequate equipment to trawl in deeper waters for shrimps. In Fig. 3 the annual landings of shrimps at Valparaíso are clearly seen to be much more important than at the other three ports, but San Antonio is now rapidly increasing in importance.

Monthly landings given in Fig. 4 are notable for the distinct reduction in catches of shrimps during the winter months, around July, and this would appear to be related in some way to the breeding period (see Section 6). However, according to the processors, there is no problem about the yield or preparation of berried shrimps, nor about their availability on the normal fishing grounds. They consider that the lowered catches result from bad winter weather, during which boats are forced to remain nearer to the ports, where however catches of langostino remain good. Maximum shrimp catches correspond in general to the lowest langostino catches, and occur particularly in summer (December to February) when the langostino are moulting (Fig. 4).

The standard technique of assessing availability of stocks in different years, by relating catches to some unit of effort, is here complicated by a number of factors. The vessels contributing to the annual effort may not be identical from year to year, and their wide range of size and fishing power has already been stressed (Table I). The problem of mixed catches needs careful consideration, since shrimps may be taken whilst trawling primarily for langostino, and vice-versa. To date, the only available statistic of effort has been the number of trips, but, with the exception of Coquimbo, there has been no information on the duration of each trip, the number of hours of trawling or the fishing position. Fig. 5 shows the available information on the total number of trips each year made by boats whose catches were entirely or mostly shrimps, and entirely or mostly langostino. Fig. 5B also shows how the largest part of the effort has been directed towards langostino, though the number of trips for shrimps rose from under 30 percent before 1962 to over 40 percent in 1966. The percentage of shrimps in the total catch followed the effort figures quite closely. In 1963 the effort for both species (Fig. 5A) and the catch of shrimps (Fig. 2) were greatly reduced when boats were concentrating on hake. In Fig. 6, which expresses the catch per trip for shrimps and for langostino for the same boats as in Fig. 5, it can be seen that although the effort for both species during 1963 was low (Fig. 5A) the catch per trip was unusually high. This is unlikely to have resulted from longer trips during 1963, because although the number of trips per month was slightly less than in previous years (Table II), it was slightly more than in following years. There is no evidence on which to decide whether the higher catches were related to reduced total effort. The catch per trip for both species apparently continued to decline slightly between 1964 and 1965 although a change in the source of data (see below) may have affected their comparability. Between 1965 and 1966 the number of trips for shrimps and the catch both increased, but both were reduced for langostino (Figs. 2 and 5). The catch per trip for both groups, however, increased from 1965 to 1966 (Fig. 6). Despite a big increase in the number of trips in 1961 (Fig. 5), there was no increase in the year's catch of langostino, and a decrease in the year's catch of shrimp (Fig. 2). Fig. 6 shows that the catch per trip of both species was greatly reduced. It is possible that there is some connection between the lowered catches and the severe earthquake and tidal wave of May 1960.

TABLE III

Annual landings of shrimps and langostinos by ports 1952 – 1966. Data from the Department of Fish and Game (1952 – 1964) and IFOP (1965 – 1966, in metric tons)

Fig. 4 Total landing by months of shrimps and langostino; data from the Department of Fish and Game.

Fig. 5A Number of trips per year for shrimps and langostino, separately and in total, for boats whose catches were entirely or mostly shrimps and entirely or mostly langostino. (Source of data as in Fig. 3).

Fig. 5B The percentage of total trips which were made for shrimps, and the percentage of shrimps in the total catch. (Source of data as in Fig. 3).

In consideration of catch per unit of effort, the influence of improved vessels and gear, of improved techniques and experience of crews, and of improved collection of statistics must be considered. For example, in January 1965, IFOP established fulltime collectors of statistics at the three major ports; Quintero is covered also by the collector at Valparaíso. The collectors are also responsible for measurements and standard biological observations. In representing catch and effort statistics, any marked change in the method of collection has been noted, and, where possible, a comparison made with the result from the method used previously (Fig. 2). This assumes particular importance when comparing data for consecutive years, as in Figs. 2, 5 and 6, and should be considered when deciding whether the decrease in catch per trip between 1964 and 1965 (Fig. 6) is a real one. Up to January 1967, the length of trip was recorded regularly only at Coquimbo, but, since this will clearly be an important consideration in any further assessment, a concerted attempt will be made in future to obtain the exact length of each trip as well as the number of hours occupied in trawling.

The data from Coquimbo for 1965 and 1966 have been analyzed separately to give some idea of the difference in result between catch per trip and catch per day, using trips for which landings were composed solely of shrimps. Table IV demonstrates clearly the errors which would be inherent in comparing annual catch per unit of effort for the total fleet without an understanding of the variation in catch per unit of effort between months and between vessels. Differences between catch per trip and catch per day at sea are small for Coquimbo (Table IV and Fig. 7) because the majority of boats were at sea for no more than one day.

None of these data differentiate between whole and part days spent at sea, and the amount of time spent actually trawling is not known. Clearly such information is desirable for all ports.

Coombs (1966) has suggested a method for separating effort in a mixed fishery for crabs and lobsters, but some information was lost due to the pooling of data over weekly periods. Saetersdal, Henríquez and Sanhueza (MS) however attempted to obtain a basis for comparison of catch per unit of effort in the mixed shrimp-langostino fishery by selecting firstly boats with a history of fishing primarily for shrimps and langostino rather than for hake. From these the average number of trips per month was calculated for each year as an index of the length of voyage, and it was concluded that there was no consistent change in this figure between 1960 and 1965 for Coquimbo and Quintero. There was, however, a marked decrease in the number of trips per month from Valparaíso between 1962 and 1966 (Table II), and, since the main bulk of landings came from this port, this is consistent with the increasing catches per trip (Fig. 6). A separation was then made into (A) trips yielding catches composed only of shrimp or only of langostino and (B) those in which catches were mixed but predominantly either shrimp or langostino. Some errors may have been introduced here in separating majority catches merely on a weight basis, without considering the fact that for equal weights shrimps command a higher price than langostino. The proportion of the annual effort i.e. in number of trips, expended on catches classified under (B) was calculated using the ratio of shrimp catch to total catch of these boats. The results were expressed for individual vessels as average catch per trip per year. The results obtained from (A) were so little different from (A) plus (B) that the latter were used for the results shown in Table V. The same authors made an attempt to find a convertible index of fishing power but were defeated by the heterogeneity of the fleet with regard to type and equipment. The comparison between years had therfore to be based on single identified vessels (Table V A and Fig. 8).

Fig. 6 Mean catch per trip for each year from 1960 to 1966 for the same boats as in Fig. 5, showing shrimp and langostino catches separately and combined. (Source of data as in Fig.3).

Fig. 7 Comparison of mean catch per day and mean catch per trip each month during 1965 and 1966 for Boat II at Coquimbo, together with mean number of days per trip.

TABLE IV

Mean catch per day, catch per trip and number of days per trip each month during 1965 and 1966 from Coquimbo (unit 100 kgs)

a : Catch per day
b : Catch per trip
c : Mean days per trip

The results for Coquimbo and Quintero show little change between 1960 and 1966, but from Valparaíso the level of catches was much more variable between boats, reflecting the heterogeneity of the fleet. One boat (number XI) showed a dramatic increase from 1960 to 1966, while another (XIII) showed the same increase in only two years. It is known that the captain of boat XI is the most skilful in the fleet, and this increase could reflect his increasing efficiency, or alternatively, there was a marked decrease in the number of trips per month from 1964 to 1966 of both vessels (Table II), so that the results may merely represent longer trips with larger catches per trip. This emphasizes the need for more information on the composition of trips.

Similarly, examination of monthly catch per trip on a fleet basis is not very meaningful without taking into consideration the different boats fishing each month. A comparison of monthly catch per trip between years is made difficult by the fact that there is no fishing for shrimps in some months, and there is no constancy about the particular months fished by any boat. Even for one boat there was no consistent pattern of monthly catch per trip from year to year.

In order to obtain an average value of annual catch per trip unweighted by the actual landings, Saetersdal, Henríquez and Sanhueza (MS) expressed the catches per trip of individual vessels in 1965 and 1966 as percentages of the catch per trip in 1964 (Table V B). There was no consistency in the pattern of increase or decrease in catch per trip between 1964 and 1966, and, excluding boat XIII, the mean result varied little between the 3 years. As is usually the case, the use of percentages showed a great tendency to bias, and the results obtained from averaging the original catch per trip figures seem to be more representative (Table V A).

5 SIZE AND SEX COMPOSITION

In March 1965, IFOP commenced sampling commercial landings of shrimps and langostino for size and sex composition. By July 1965, regular monthly samples were being measured by permanent collectors at Valparaíso and Coquimbo, and this was extended to San Antonio in January 1966. Measurements of carapace length from the eye socket to the posterior mid-dorsal edge of the carapace are made using dividers and a steel rule, and recorded to the nearest half millimetre below.

The proportion of male shrimps in the samples was usually less than 50 percent (Fig. 9 A), averaging 32 percent for 1965 samples taken mostly at Coquimbo, and 29 percent for 1966 samples from all three ports. These figures included a maximum of 64 percent (Coquimbo in November 1965) and a minimum of 3 percent (Valparaíso in July 1965 and Coquimbo in March 1966). This compares with the average of 25 percent (50 percent in October and 0 percent in June and July) obtained by Bahamonde and López (1960) from Santiago market samples examined during 1958 and 1959 (Fig. 9 B). Both the latter samples, and those from Coquimbo in 1966 contained very small numbers of males in March, June and July, but this did not apply to Valparaíso, or to the average. At the present state of knowledge, no speculation will be made as to the cause, remembering that samples have been grouped here according to port of landing, and not according to fishing area, which should be done at a later stage.

The size compositions of males, total females and ovigerous females are given in Fig. 11. The modal and average sizes for all three are greater for shrimps landed at Valparaíso than Coquimbo (San Antonio is most like Valparaíso) and this must be remembered when comparing the average size in different years as a way of indicating intensity of fishing. Saetersdal, Henríquez and Sanhueza (MS) used this method to compare the recent size composition with that given by Bahamonde and López (1960), but they were careful to select samples taken from a comparable annual period (June 1965 to June 1966) and from a similar area (Valparaíso and San Antonio) to that represented in the 1958–59 samples. They concluded that the size compositions for the two periods were so similar that there was no evidence that the fishery had significantly increased the total mortality of the stock.

TABLE V

(A), Mean annual catch per trip (tons) for catches consisting entirely or principally of shrimps. (B), values for 1965 and 1966 as percentage of 1964 values. Boats from Valparaiso. (After Saetersdal, Henriquez and Sanhueza, MS)

Fig. 8 Mean annual catch per trip of shrimps for identified vessels from Coquimbo, Valparaíso and Quintero (after Saetersdal, Henríquez and Sanhueza, MS). (Source of data as in Fig. 3)

Fig. 9 Percentage of males in monthly samples (A) from 3 ports in 1965 to 1967, and (B) in 1958 to 1959 (data from Bahamonde and López, 1960).

Fig. 10 Percentage of females which were ovigerous in monthly samples. (A) and (B) as in Fig. 9.

Fig. 11 Mean carapace length composition of males, total females and ovigerous females in samples taken during 1966 at Coquimbo and Valparaiso, shown as percentages in each millimetre group. Mean carapace lengths are also shown.

Fig. 12 Relationship between total weight and carapace length for males, non-ovigerous and ovigerous females from Valparaíso, 15 June 1966.

Fig. 13 Part of a 30-min haul (about 800 kg) of Heterocarpus reedi (with Paralichthys sp.) taken on M/S TIBERIADES off Talcahuano in November 1966. (Photo: G. Henríquez).

Fig. 14 A day's catch of Cervimunida johni by a commercial vessel off Coquimbo. (Photo: M.N. Mistakidis).

6 BREEDING

Records were kept of the numbers of ovigerous females in the samples. Fig. 10A is composed from the available information from samples from various sources in 1965 and 1966. Although there is some slight difference in the positions of the modes from samples from the different ports, which would warrant a more detailed examination of the precise location of samples, the general pattern is clear; ovigerous females showed a maximum (up to 100 percent) during the winter months and a minimum (down to 0 percent) in the summer. The results of Bahamonde and López (1960) were very similar (Fig. 10 B). Samples taken in exploratory cruises off Talcahuano gave average percentages of ovigerous females more like Valparaíso than Coquimbo i.e. 60 percent in November 1965, 2 percent in March 1966 and 45 percent in November 1966, supporting the suggestion of later occurrence of ovigerous females on more southerly grounds. It should be mentioned however that the results from individual samples in November 1965 were extremely variable, ranging from 12 to 100 percent. The fact that more than 50 percent of female shrimps are ovigerous during half the year was used by Saetersdal, Henríquez and Sanhueza (MS) to point out the impracticability of the regulation prohibiting the taking of shrimp and langostino with visible eggs. Such specimens would be unlikely to survive if returned to the sea. During the peak period of ovigerous females, all females in the July sample from Coquimbo, from a carapace length of 20 mm, carried eggs. At Valparaíso the smallest ovigerous female was 23 mm, and at San Antonio 19 mm.

Bahamonde (1958) made estimates of the number of eggs peŕ female shrimp over a range of carapace lengths. The numbers ranged from 1,700 per 25 mm female, to 3,320 at 35 mm.

7 LENGTH/WEIGHT RELATIONSHIP

Brief mention must be made here of the relationship between length and weight which is an essential part of any population study. Preliminary results (Fig. 12) suggest that there is no difference between males and non-ovigerous females, but that egg-bearing females are heavier for the same carapace lengths. Summer and winter samples from Valparaíso gave similar results, and there was no difference between summer samples from Valparaíso and San Antonio. Regression lines (Hancock and Edwards, 1966) have not yet been fitted.

8 SUMMARY AND CONCLUSIONS

The information presented and reviewed represents observations to date relevant to a stock assessment of the Chilean shrimp fishery, and needed for the framing of regulations. It includes data on the distribution of the fished and unfished stocks, on catch and effort, on size and sex composition and incidence of berried females, and on the relationship between carapace length and body weight. The data are by no means complete, but this analysis has served to show their shortcomings, and already measures have been taken for their improvement. Unfortunately there is no information on growth rates.

More detailed records of effort are required, but it seems clear that there has been no consistent change in the average catch per trip. The gradual decrease in the number of trips per month from Valparaíso is, however, indicative of longer trips, and in 1966 there was a substantial increase in the average catch per trip from this port. This coupled with a greater total effort gave rise to a marked increase in total landings during 1966. It seems reasonable, therefore, to agree with the conclusion of Saetersdal, Henríquez and Sanhueza (MS) that not only does the fishery not seem to have had a great effect on the stock, but that in view of the discovery of stocks as yet lightly fished, there could be a further increase in the Chilean catch of shrimps.

9 REFERENCES

Bahamonde, N., 1955 Hallazgo de una especie nueva de Heterocarpus en aguas chilenas: H. reedi n. sp. Investnes zool.chil., 2:105–14

Bahamonde, N., 1958 Sobre la fecundidad de la gamba o camarón nailon (Heterocarpus reedi Bahamonde). Investnes zool.chil., 2:259–64

Bahamonde, N., 1963 Decápodos en la fauna preabismal de Chile, Noticiario mens. Mus.nac.Hist.nat., (81):10 p.

Bahamonde, N., and M.T. López, 1960 Observaciones sobre la época de desove de la gamba (Heterocarpus reedi Bahamonde) Revta Univ.,Santiago, 44(5):39–41

Coombs, R.F., 1966 Separating true lobster and crab catch per unit effort in a mixed fishery. ICES, C.M., Shellfish Committee (11) (mimeo)

Hancock, D.A., and E. Edwards, 1966 The length/weight relationship in the edible crab (Cancer pagurus). ICES, C.M., Shellfish Committee (18) (mimeo)

Lindner, M.J., 1957 Survey of shrimp fisheries of Central and South America. Spec.scient. Rep.U.S.Fish Wildl.Serv., (235):166 p.

López-Capont, et al., 1965 Industrialización de langostinos y camarones en Chile (Fresco, Congelado, conserva, seco, preparaciones especiales, etc.) Publnes Inst. Fom.pesq., (8):72 p.

Mistakidis, M.N., and G. Henríquez, 1966 Report on the shrimp-langostino exploratory survey in the Constitución - Isla Mocha area, October - November, 1965. Publnes Inst.Fom.pesq., (16):37 p.

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