Short time variation in catch per unit effort (CPUE) of shrimp (Pandalus borealis) at the Flemish Cap

by

Jón Kristjánsson

Fiski- Fisheries Management, Reykjavík Iceland.

E-mail: jon@fiski.com - URL: http://www.fiski.com

12. January 2001

Introduction

Data on catch per unit effort (CPUE) are widely used in management and stock assessment. At the Flemish Cap, CPUE data from surways have been used to estimate the age/size composition of the stock and relative abundance from year to year in order to give advice on management of the stock. This study is to show the variability of the parameter and leads to the conclusion that great care has to be taken when surway results are used to intemperate the status of the stock.

Materials and methods

Data on shrimp were collected by the author on board Icelandic and Estonian shrimp trawlers on several occasions in the period November 1995 - April 2000. The shrimps were measured to the nearest 0.1 mm, sorted into 0.5 mm length classes (OCL) and separated into sex categories, i.e. males, transitional, and females. Information on location, time of day, duration of, catch in and mean size (count) of shrimp in each tow was obtained from the trawler's logbooks.

Results

Generally, there were three hauls of 5-7 hours duration each day. The morning haul was from 03-12 local time, afternoon haul from 12-20.30 and the night haul until 03. Information from the logbooks was used to compute the effective trawling time, which varied from 61-81% of the time spent at sea, catch per unit effort (CPUE) in morning-, afternoon- and night hauls and the mean CPUE for each day. The results are shown in table 1. The table covers 18 fishing trips of three different trawlers, A, B and C in the period January 1996 - August 1997. All were single trawlers. The average catch per unit effort is similar in the morning and afternoon but very much lower at night. The night CPUE is only 1/2 - 1/5 of the daytime values for the whole period.

Table 1. Duration of fishing trips, effective trawling time in each trip, CPUE's at different times of the day in the period 1.1. 1996 to 1.9. 1997, by three shrimp trawlers at the Flemish Cap.

A graph of the CPUE's by time is shown in fig. 1. Note that the time intervals are not real scale.

Figure 1. Plot of the CPUE data in morning- afternoon and night hauls from table 1.

In order to take a closer look at details in the CPUE, an example is taken from one particular fishing trip on trawler A 23/6 - 8/8 1997. The trawl was shot 107 times, 76 times during the day and 31 times during the night. Distribution of the CPUE night and day and locations of the hauls are shown in fig 2. The fishing took place on the SW, W and N part of the Cap. Two of the hauls were in the SE part. It often happens during practical fishing that night time are used to travel long distances to explore distant areas. Also, night time is often used for major trawl repairs, then loss of catch during repair or overhaul is minimized.

Figure 2. Distribution of CPUE in, a) day-hauls, and b) night- hauls. Location of all the hauls in the fishing trip by trawler A 23/6 - 8/8 1997 is also shown, c).

Figure 3 shows the CPUE in 45 consecutive tows from the trip, also the average weight (no/kg) of shrimp from each tow and the depth in meters. This is typical for the overall picture and is related to the distribution and behaviour of the shrimp at the Flemish Cap. CPUE is lower at night and shrimps are then smaller on the average. Shrimp move up from the bottom during the dark hours to feed on plankton, particularly the small shrimps. This is supported by the findings of Ivanova[1], who reported higher contents of planktonic food remains in the stomachs of males than in larger females.

Figure 3. 45 consecutive tows from trawler's A trip 23/6 - 8/8 1997. Note the variation in CPUE, it is low during the night, high during the day. Size of shrimp varies by time of the day and by depth. Note that the depth perspective is from the surface to the bottom (scale is reversed).

Discussion

It is known that shrimps migrate up from the bottom during the night to feed on plankton and that small shrimp travel more than the larger ones. This is probably due to the the fact that the volume surface ratio makes it easier to "float" in open water and is therefore less energy consuming. The largest females probably sit on the bottom most of the time. Often, nearly all small shrimp disappear from the night catch, for again to show up in the catch during the daytime. It thus seems that they all migrate up to higher water layers but it is not known how large part of the population goes down again. Perhaps only part of the stock is fishable at all times?

As long this is not known, population estimates relying on CPUE must be used with great care. It seems to me that all stock estimates regarding shrimp are on the conservative side. Especially should short time assessments be used with care. Night sampling in an areas with small shrimp will give misleading results.


[1]   Ivanova, I.V. 2000. About Pandalus borealis (Kroyer, 1848) nutrition on Flemish Cap. NAFO SSC. Doc. 00/65.



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