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SPC/lnshore Fish. Res./BP,34 6 March 1988
ORIGINAL : ENGLISH
SOUTH PACIFIC COMMISSION
WORKSHOP ON PACIFIC INSHORE FISHERY RESOURCES (Noumea, New Caledonia, 14 - 25 March 1988)
PROGRESS REPORT ON A PROGRAMME TO ASSESS BOTTOM FISH STOCKS ON SEAMOUNTS IN TONGA
by
S.A. and V.A. Langi
Fisheries Division, Kingdom of Tonga
ABSTRACT
The i n i t i a l 9 month's data from a 5 y e a r s t o c k a s s e s s m e n t of t h e d e m e r s a l f i s h e r y of Tonga's seamounts was analysed, c h i e f l y t o check sample design and to e s t a b l i s h parameters upon which t o base future comparisons. The data f a l l s i n t o 2 c a t e g o r i e s , c a t c h - p e r - u n i t e f f o r t and length/weight measurements. A summary of the r e s u l t s d e t a i l e d from each data s e t i s as f o l l o w s .
a) CPUE
25% of a l l d e e p - b o t t o m f i s h i n g e f f o r t i s c a p t u r e d by t h e s a m p l i n g method.
The t o t a l catch for the f i r s t half of 1987 was 235 t . A t o t a l catch of 470 t per annum i s p r e d i c t e d .
Maximum s u s t a i n a b l e y i e l d i s about 325 t p . a .
Average CPUE = 1 . 5 f i s h or 6.71 kg per standard f i s h i n g r e e l per hour. b) Length/weight data
Length/weight r e g r e s s i o n s for 7 important s p e c i e s were s u c c e s s f u l l y c a l c u l a t e d .
A t a b l e of growth and m o r t a l i t y parameters was drawn up f o r t h e s e s p e c i e s .
Y i e l d - p e r - r e c r u i t (Y/R) a n a l y s i s shows t h e m a i n e x p o r t s p e c i e s , Pustippmoides f i l a m e n t o s u s , i s only being fished at 46% of i t s p o t e n t i a l Y/R but the other major s p e c i e s are close t o t h e i r p o t e n t i a l .
It was concluded that sample design i s b a s i c a l l y adequate although more length measurements c o u p l e d w i t h s u c c e s s f u l d e p l e t i o n experiments are needed t o
estimate Bo, the i n i t i a l stock s i z e .
SPC/lnshore Fish. Res./BP.34 Page 2
1. INTRODUCTION
In October 1986, following an SPC/FAO/UNDP workshop on stock assessment, the Tonga Fisheries Division began a 5-year programme to assess the stocks of bottom fish of the seamounts in Tonga. The design of this programme was directly based on ideas learnt at the workshop in Noumea. (Walters & Hilborn 1986).
Data collected during the first 9 months of the programme were analysed at the NMFS Honolulu Laboratory under the supervision of Dr.J. Polovina, in order to highlight any problems in sampling design, and to establish parameters upon which to base future comparisons.
This report gives the results of this preliminary analysis, based mainly on the first 9 months of data. The data falls into two broad categories;
1) Catch and effort information 2) Length and weight measurements These form the 2 main sections of this paper.
2. BACKGROUND 2.1 Fishing Grounds
The majority of seamounts in Tonga lie on a submerged ridge, running in a NE-SW line parallel to the Tonga ridge which carries the 170 islands of Tonga (Figure 1). As an estimation of size of fishing grounds, the length of the 200 metre isobath was recorded as 930 nautical miles. However, the hydrography of many areas is imcomplete and new seamounts are still being discovered. 2.2 The Fishery
The commercial development of the f i s h e r y , with i t s t a r g e t species of snappers and groupers (f. Lutjanidae, Serannidae), began in 1980 after the v i s i t s of t h e SPC Deep Sea F i s h e r i e s Development Project in 1978 and 1979, and the establishment of the UNCDF boatbuilding scheme. In July 1987, 37 boats were engaged in bottom f i s h i n g . By December 1987, t h i s number had increased t o 45. All f i s h i n g boats use the FAO-designed Western Samoan wooden r e e l , with a m u l t i p l e - h o o k t e r m i n a l r i g . (For a d e t a i l e d d e s c r i p t i o n of t h i s f i s h i n g technique, see Mead (1979)). The depth of fishing v a r i e s from 50 - 400 metres.
SPC/Inshore Fish. Res./BP.34 Page 3
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9 >l-oo <a.-oePRw 2^S
M
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a
* •7 +a
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Figure 1 : Map of Tonga showing seamount locations and deep-bottom catch rates (number of fish
per reel-hour for the commercial fishery (All species combined)
SPC/Inshore F i s h . R e s . / B P . 3 4 Page 4
CATCH & EFFORT DATA - METHODS
3 . 1 Data C o l l e c t i o n
D e t a i l s of catch and e f f o r t a r e recorded in 3 c e n t r e s : - Tongatapu, Vava'u and N i u a t o p u t a p u . In T o n g a t a p u a t t e m p t s a r e made t o r e c o r d every t r i p by every b o a t , but only 66% coverage has so f a r been a c h i e v e d . In Vava'u t r i p s a r e o n l y recorded q u a r t e r l y . I n Niuatoputapu every t r i p i s r e c o r d e d from t h e
s i n g l e deep-bottom f i s h i n g boat based t h e r e . Elsewhere no r e c o r d i n g has t a k e n p l a c e due t o lack of manpower.
Table 1 shows the number of b o a t s in each major c e n t r e , and t h e p e r c e n t a g e of f i s h i n g e f f o r t r e c o r d e d .
TABLE 1 : PERCENTAGE OF FISHING EFFORT CAPTURED BY SAMPLING METHOD CENTRE % SAMPLED NO. BOATS
T o n g a t a p u 6 6 . 6 11 Niuatoputapu 100.0 1 Vava'u 5.6 16 H a ' a p a i 0.0 6 'Eua 0.0 3 T o t a l 2 4 . 8 37 T a b l e 1 shows t h a t a b o u t 25% of a l l f i s h i n g e f f o r t i s b e i n g s a m p l e d . T h e r e f o r e , t o e x t r a p o l a t e e s t i m a t e s of catch or e f f o r t t o Tonga as a w h o l e , r e c o r d e d f i g u r e s must be m u l t i p l i e d by 4 . C a t c h p e r u n i t of e f f o r t (CPUE) i s r e c o r d e d as numbers of f i s h p e r s p e c i e s caught per r e e l - h o u r . Although counts a r e made of each s p e c i e s of f i s h i n t h e c a t c h , only the f o l l o w i n g 7 major s p e c i e s shown i n Table 2 below a r e used in t h e d a t a a n a l y s i s :
TABLE 2 : MAJOR SPECIES TAKEN IN THE FISHERY SP1 P r i s t i p o m o i d e s f i l a m e n t o s u s SP2 p , f l a v j p i n n i s SP3 E t e l i s corugcans SP4 E. carbunculug SP5 E p i n e p h e l u s morrhua SP6 E> s e p t e m - f a s c i a t u s SP7 L e t h r i n u s chrvsostomus 3.2 R e s u l t s 3 . 2 . 1 Geographical v a r i a t i o n s in CPUE
S p a t i a l maps were drawn showing t h e CPUE (numbers of f i s h per r e e l - h o u r ) f o r each l o c a t i o n . F i g u r e 1 shows t h e CPUE for a l l s p e c i e s combined. The h i g h e s t c a t c h r a t e s a r e from a r e a s f i s h e d from Tongatapu with a maximum of 2.3 c l o s e t o 'Ata i s l a n d . I t i s i m p o r t a n t t o n o t e t h a t more l o c a t i o n s around Tongatapu h a v e b e e n r e c o r d e d t h a n e l s e w h e r e i n t h e Kingdom due t o c o n c e n t r a t i o n of sampling e f f o r t . ( p a l u h i n a ) ( p a l u s i o ' a t a ) ( p a l u t a v a k e ) ( p a l u malau) ( n g a t a l a ) (mohuafi,tonu or mala) (manga)
SPC/Inshore F i s h . Res./BP.34 Page 5
The CPUE data were summarised by the grid squares shown in f i g u r e 1 . R e s u l t s are shown in figure 2. Grid square 13 shows the h i g h e s t catch r a t e of more than 2 f i s h p e r - r e e l hour. Grid squares 9 and 16 are nearly as h i g h , w h i l e g r i d square 2 i s the l o w e s t . These f i g u r e s w i l l be monitored on an annual b a s i s .
3 . 2 . 2 Total catch
Figure 3 shows the recorded catch (numbers of f i s h ) for the whole of Tonga for each quarter from November 1986 t o July 1987. These values must be m u l t i p l i e d by 4 t o e s t i m a t e the t o t a l c a t c h .
At the time t h a t f i g u r e 3 was prepared in August 1987, the data s e t did not span a f u l l y e a r , an estimate of annual catch was made by doubling the t o t a l catch e s t i m a t e for the 6 months from January t o June 1987. On t h i s b a s i s , the p r e d i c t e d annual c a t c h would be 1 0 7 , 4 9 6 f i s h , which, using the c a l c u l a t e d average f i s h weight of 4.365 kg, g i v e s a predicted t o t a l landing of 470 t .
At the end of 1987, a t o t a l of 40,982 f i s h had a c t u a l l y been sampled. Using t h e same e x t r a p o l a t i o n t e c h n i q u e ( m u l t i p l y i n g by 4 ) , the estimated t o t a l mumber of f i s h caught in 1987 would be 163,928, or 716 t . This f i g u r e i s 52% h i g h e r than t h e p r e d i c t e d 470 t and a 123% increase on the estimated 1986 catch of 321 t . These i n c r e a s e s are no doubt at l e a s t p a r t l y a s s o c i a t e d with the i n c r e a s i n g number of v e s s e l s entering the f i s h e r y .
3 . 2 . 3 Production Curves
A plot of catch against effort, using data extrapolated to the whole of Tonga was begun (figure 4 ) . From the small amount of data presently available,
catches appear to increase as effort increases, indicating that the fishery is still on the ascending limb of the catch curve. More data points added later will give a more meaningful picture.
3.3 Discussion
3.3.1 Sampling methods
Analysis of CPUE data so far has shown that the sampling method is adequate to obtain the information required to estimate total catch and effort in Tonga. The data collected will allow yearly monitoring of catch rates of individual species and enable patterns of over or underexploitation to be detected. As the study progresses, more information will emerge on levels of exploitation, the abundance and distribution of the resource, sustainable yields, species composition, etc.
3 . 3 . 2 Catch r a t e s
The t o t a l c a t c h r a t e f o r Tonga may be c a l c u l a t e d as t o t a l number of f i s h caught d i v i d e d by t o t a l e f f o r t in r e e l - h o u r s . This g i v e s a v a l u e of 1.5 f i s h / r e e l - h o u r , or 6.7 k g / r e e l - h o u r at an average f i s h weight of 4.365 kg. This i s compared with catch r a t e s given for 10 P a c i f i c I s l a n d c o u n t r i e s in Table 3 .
SPC/Inshore F i s h . R e s . / B P . 3 4 Page 6 9 0 0 0 -8000 7000 6000 5000 4000 3000 2000 • 1000 •
o-Catch (thousands of fish) Nov-Oec 1986 (2 months only) Jan-Mar 1987 APr"J u n 1 9 8 7 Quarter July-Sep 1987 (1 month only) F i g u r e 2 : T o t a l deep-bottom f i s h c a t c h i n Tonga 2 . 0 -1.8 • 1.6 -1.4 • 1.2 • 1.0 • 0.8 • 0.6 0.4 0.2 0.0 J CPUE (no/ reel-hour) 5 6 7 8 9
6rid square number
13 14 15 16
SPC/Inshore Fish. Res./BP.34 Page 7 M a v e a a 3 O r 13 12 11 -10 9 8 7 6 -S A 5 2 1 -0
Catch versus Effort (Reel Hours)
All spaela* for all of Tonga
Apr, Way, June
•
July. Jan. Feb, Mar Nov 4 D«e
1 r 1 1 1 1 1 1 1 r—
2 4 6 8 10 (Thousands)
Effort (Raal Hours)
12
Figure 4 : P l o t of catch vs e f f o r t ( r e e l - h o u r s )
TABLE 3 : HAND-REEL CATCH RATES (IN KG/REEL/HOUR) BY SPC BOTTOM FISHING PROJECTS AT DIFFERENT PACIFIC ISLAND LOCATIONS.
(from Crossland, 1980)
American Samoa 4.4
Kosrae 9.6 New Caledonia 7.6
New Hebrides (Tanna) 3.1
Niue (1978) 2.8 Niue (1979) 7.0 AVERAGE Palau Papua Tonga Tonga Truk
Yap
: 5.3 New Guinea (1978) (1979)3.3
4.9
3.6
5.7
4.1
6.9
The average figure for the Tongan f i s h e r y , 6.7 k g / r e e l - h o u r , i s higher than the average of 5.3 given in the t a b l e , and a l s o higher than the 1978 and 1979 f i g u r e s for Tonga (Mead 1 9 7 9 ) . However, t h e s e e a r l i e r f i g u r e s came from e x p l o r a t o r y or f i s h e r y t r a i n i n g e x e r c i s e s in Tonga, not from a commercial f i s h e r y . The d i f f e r e n c e in c a t c h r a t e s could be a t t r i b u t a b l e t o a wide v a r i e t y of d i f f e r e n c e s in f i s h i n g pattern and methods of o p e r a t i o n .
SPC/Inshore Fish. Res./BP.34 Page 8
3.3.3 Sustainable yield
I t is too early in t h i s study t o accurately assess sustainable y i e l d from the
bottom fishery in Tonga, but i t is possible to make a very rough estimate from
the data gathered so f a r . A recent approach t o estimating sustainable y i e l d
was t h a t used in Hawaii (Ralston & Polovina 1982) and the Mariana Islands
(Polovina et a l , 1985). Here the length of the 100 fathom (200m) isobath was
used t o predict the annual sustainable yield of bottom f i s h . An estimate of
220-270 kg per nautical mile of 100-fathom contour was obtained.
Using t h i s estimate, Tonga's 930nm of 200m contour would yield 153 to 260 t
per year. We may wish to add on an a r b i t r a r y 25% to cover as yet undiscovered
seamounts, giving a figure of 191 t o 325 t p . a . F i j i ' s F i s h e r i e s D i v i s i o n
(1987), following Brouard (1984) who worked on Vanuatu's bottom f i s h , doubled
Polovina
1s estimate before adding for unknown seamounts. This method allows
for t h e f a c t t h a t , in an u n e x p l o i t e d or very l i g h t l y exploited stock, the
standing biomass can be expected t o generate considerably higher catch r a t e s
in the i n i t i a l development of the fishery than w i l l be sustained in the longer
term. Applying the method to Tonga's case gives yield estimates of between
382 and 650 t per annum, but these could only be expected t o hold for t h e
f i r s t year or two of the f i s h e r y ' s operation.
As noted previously, t o t a l deep-bottom fish landings in Tonga were estimated
at 321 t and 716 t in 1986 and 1987 r e s p e c t i v e l y . Tonga's present catches
could t h e r e f o r e be f a s t approaching or exceeding t h e s u s t a i n a b l e y i e l d .
Caution is thus recommended in allowing i n c r e a s e s in f l e e t size u n t i l more
data i s c o l l e c t e d . If properly managed at stable levels of e x p l o i t a t i o n , the
bottom fishery could continue successfully for many years. According to
Munro ( 1 9 8 6 ) , when t h e maximum annual h a r v e s t i s being taken, CPUE w i l l
decline to one half or one t h i r d of i t s i n i t i a l v a l u e s . If we apply t h i s
g e n e r a l i s a t i o n t o Tonga, using the maximum CPUE value of 2.3 f i s h / r e e l - h o u r
( e s t i m a t e d for grid square 13 in f i g u r e 1 ) , then the following p i c t u r e
emerges:-I n i t i a l CPUE = 2.3 fish per reel-hour, or 10.5 kg per r e e l - h o u r . At
MSY, CPUE f a l l s t o , say, 3.5 kg per reel-hour. If MSY = 325 t (based on
Polovina's figures plus an a r b i t r a r y 25% then 325 t /3.5 kg gives the
equivalent of 93,000 r e e l - h o u r s of e f f o r t . The average number of of
reel-hours per t r i p is 73, so 93,000 reel-hours / 73 = 1274 t r i p s . The
average number of t r i p s per year per boat is 37, therefore 1274 / 37 =
34 boats. That i s , i t i s estimated that 34 boats w i l l be required to
harvest the maximum sustainable y i e l d . At present, there are 45 boats in
the bottom fishing f l e e t .
3.3.4 Depletion estimates
Attempts were made to carry out "Depletion" or "Intensive Fishing" experiments on 3 seamounts by the government vessel the M.V. Takuo during November and December 1986. The aim of this type of experiment is to fish a seamount intensively, over a short time period, until catch rates drop, thus showing depletion in fish stocks. By carefully recording hours of effort and catches, graphs can be constructed to estimate the initial size (Bo) of the fish stock on that seamount, using the Leslie model (Ricker 1975).
SPC/lnshore Fish. Res./BP.34 Page 9
Unfortunately, these attempts were mainly unsuccessful. The seamounts were larger than expected and a longer time was required to deplete them using one boat. Subsequently the research section lost the use of the boat and no further experiments could be conducted.
Without the use of a boat, further work can be done on depletion estimates only with full cooperation of the commercial fishing boats. This w i l l be attempted, since an estimate of Bo needs to be obtained.
4. LENGTH - WEIGHT DATA 4.1 Methods
Length - weight data are collected in the same three centers, Tongatapu, Vava'u and Niuatoputapu. Measurements are taken q u a r t e r l y , in the months February, May, September and December. In the sampling month, one whole
catch from every boat is measured i . e . 4 catches per boat per year.
Catches from the 2 government fishing vessels, MFV Takuo and Albacore, are also recorded whenever they go fishing. These fishing trips are infrequent and inconsistent due to high running costs and other commitments on the part of the vessels.
All lengths are fork lengths, taken to the nearest centimetre. All weights are recorded before the fish are gutted and taken to the nearest 0.1 kg on 20 kg and 100 kg hanging spring balances.
4.2 Results
4.2.1 Length - weight regression
Length/weight regression analyses were successfully completed for the seven major species listed in table 2. This will enable us to avoid further weighing of these fish after the f i r s t year.
4.2.2 Length frequency distribution
Quarterly length-frequency d i s t r i b u t i o n histograms for each species were plotted. They show that we do not have enough data in a time series to be able to run the ELEFAN series of computer programmes which allow estimation of population parameters from modal progressions. We were advised by Dr. Polovina to c o l l e c t 1000 length measurements per year per species. This will s t i l l not be enough to run ELEFAN, but enough to allow reasonably reliable estimates of Z/K. More pragmatically, this is a l l it is feasible to collect without upsetting the fishermen.
SPC/Inshore F i s h . R e s . / B P . 3 4 Page 10 4 . 2 . 3 P o p u l a t i o n p a r a m e t e r s From t h e l e n g t h - f r e q u e n c y d a t a , i t was p o s s i b l e t o e s t i m a t e t h e f o l l o w i n g p o p u l a t i o n p a r a m e t e r s for each s p e c i e s : Lc Linf Lm Lmax Lbar K M F Z M/K Z/K F/M Y/R
= the smallest length fully represented in the catch) = asymptotic length (von Bertalanffy growth equation)) = length at onset of sexual maturity)
= maximum length recorded)
= mean length of all fish with lengths greater than Lc) = growth factor (von Bertalanffy growth equation)) = coefficient of natural mortality)
= coefficient of fishing mortality)
= coefficient of total mortality (sum of F and M)) = ratio of natural mortality to growth)
= ratio of total mortality (F + M) to growth) = ratio of fishing mortality to natural mortality) = yield per recruit (expressed as a fraction of Winf,
the asymptotic weight) (Beverton & Holt 1966))
To compute Linf and Z/K a modification of the Beverton & Holt equation was used (Wetherall at al. in press). Z/K is assumed to equal M/K if the stocks are lightly exploited. This could not be assumed in our case since commercial fishing has occurred since 1980. For M/K values, data obtained from the Marianas (Polovina and Ralston 1986) were used where possible. Elsewhere, a value of M/K = 2 . 0 was used (Ralston 1986).
To compute F/M the following equation was used:
(Z/K)/(M/K) =* (F+M)/M = (F/M) + 1
Lm was e s t i m a t e d t o be 50% of Linf (Grimes 1 9 8 6 ) .
For a p r e l i m i n a r y e s t i m a t e of K, Linf was used ( f o l l o w i n g Manooch ( 1 9 8 6 ) ) as f o l l o w s :
Log 10 (K) = 1.098 - 0 . 6 5 8 Log 10(Linf (mm))
Only 4 s p e c i e s , (SP1-3 & SP5) had enough o b s e r v a t i o n s t o allow Linf and Z/K t o be d i r e c t l y c a l c u l a t e d . From t h e c a l c u l a t i o n s and e s t i m a t e s , t h e f o l l o w i n g t a b l e was compiled:
TABLE 4 POPULATION PARAMETERS FOR THE SPECIES LISTED IN TABLE 2
Species Lr Lc Lmax Z/K M/K Linf
Lm
F/M
SPl SP2 SP3 SP4 SP5 SP6 SP7 56 39 44 59 48 108 44 65 43 59 70 60 120 54 75 57 96 114 80 172 76 4 . 1 1 1 . 5 3.33 -1.33 -— 2 . 5 4.6 2.2 -— 77.2 57.5 99.3 -74.2 -— 38.6 28.8 49.6 -37.1 -— 0.6 1.5 0 . 5 -— 0.16 0.19 0.13 -0.16 -_
SPC/Inshore Fish. Res./BP.34 Page 11
4.2.4 Yield per recruit
For each species the maximum yield per recruit is calculated from the Beverton & Holt equation and compared with the yield for the current size of entry to the fishery.
Where F/M could be calculated, Y/R was computed for that level of F/M. Elsewhere, a level of F/M = 0.5 was used.
Where no values of Linf were available, Lmax was used to compute the C value for the Y/R table, Lm and Lopt.
Values of M/K required for the tables were derived from the Marianas data (see previous table). Where none was available, a value of M/K = 2 . 0 was used (Ralston 1986).
TABLE 5 COMPARISON OF THE CURRENT SIZE OF FISH AT ENTRY TO THE FISHERY (Lr) AND THE OPTIMUM SIZE (Lopt) TO GAIN
MAXIMUM YIELD PER RECRUIT (Y/R).
Species Lr Lm F/M Y/R at Lr/Linf Lopt (F/M) Max Y/R P. filamentosus 56 38.6 0.6 0.46 29.3 P. flavipinnis 39 28.75 1.5 0.15 18.4 39.7 45.6 29.7 68.8 30.4
E.
E.
E.
E.
L.
coruscans carbunculus morrhua septemfasciatus chrysostomus44
59
48
108
44
49.657*
37.186*
38*
0.5
0.5
0.5
0.5
0.5
0.99 0.95 0.760.8
0.87* estimated from Lmax, not Linf
4 . 3 D i s c u s s i o n
For every s p e c i e s except E. coruscans Lc>Lm i . e . the f i s h enter the f i s h e r y at a s i z e larger than that at which they gain sexual maturity. This i s a good s i t u a t i o n , s i n c e the r i s k of driving down the spawning stock i s reduced.
Polovina (1986c) concludes that :
i f Lc>Lm then Fopt. < = 2M i f Lc<=Lm then Fopt. < « M
That i s , the optimum l e v e l of f i s h i n g mortality can be almost twice that of natural m o r t a l i t y as long as the f i s h reach maturity before they are f u l l y represented in the catch. We do not y e t know t h e m o r t a l i t y r a t e s f o r our s p e c i e s , but as long as Lc> Lm, t h e r e appears to be an inherent s a f e t y factor which provides some p r o t e c t i o n t o spawning f i s h .
SPC/Inshore Fish. Res./BP.34 Page 12
In considering yield per recruit, again a healthy picture appears. In every case the fishery can afford to take smaller fish than it is actually taking. However, the optimum length (Lopt) does not take size of maturity (Lm) into account. Obviously, if fish are taken after they reach maturity, a higher level of fishing effort can be employed without wiping out the spawning stock.
If we look at the ratio of current Y/R to max Y/R it appears that E. coruscaos and E... carbunculus are very close to their optimum yield. All other species are satisfactory except P... filamentosus which is currently producing only half its potential.
In the case of P. „ flavipinnis. a first analysis of Z/K produced a negative value. Using a different method, a value of 11.5 was obtained, which is very high. This could be due to one, or a combination, of several factors:
a) the fish are being rapidly depleted through overfishing
b) the fishermen are avoiding them, possibly due to the inconvenience of hauling small fish from deep water (P. Mead, pers.comm.)
c) pooling of sexes during sampling d) depth stratification.
The K (growth f a c t o r ) v a l u e s computed were c o n s i s t e n t l y lower than those produced from the Marianas data ( s e e Table 6) and must be viewed w i t h c a u t i o n .
TABLE 6 POPULATION PARAMETERS FOR THE SEVEN MAJOR SEPCIES CAUGHT BY HANDLINING IN THE MARIANAS
Species Von Bertalanffy growth parameters Linf. (cm) K(l/yr) Instan-taneous natural mortality (M) Age of entry t o the catch t c ( y r ) Age of maturity tm(yr)
c.
p.
p.
p.
E.
E.
lugubris filamentosus auricilla zonatus coruscans carbunculus 75.1 67.3 42.6 47.0 97.6 69.1 430 228 335 245 166 175 53 57 81 63 38 1. 4. 3. 4. 6. 3 3 6 65 2 1. 2. 2. 3, 4. 8 0 4 25 1 1.55 3.45 2.75If otoliths taken during the sampling programme can be read, then values of K for each species can be directly obtained from them. Once reliable K values are available then the M values can be calculated.
5.
SUMMARYAnalysis of the first 9 months data has shown the basic sample design to be adequate except that:
a) more quarterly length samples are required (1000 p.a. for each major species to estimate Z/K; and
SPC/Inshore F i s h . Res./BP.34 Page 13
If t h e Hawaii and Marianas e s t i m a t e s a r e u s e d , t h e t o t a l Tongan deep-bottom c a t c h of 716 t for 1987 could be over t h e ' s a f e ' e s t i m a t e d MSY of 325 t . A r e s u l t i n g drop in c a t c h r a t e s may t h u s be e n c o u n t e r e d i n t h e f u t u r e . The c u r r e n t c a t c h r a t e of 6 . 7 1 k g / r e e l - h o u r i s h i g h compared t o c a t c h r a t e s o b t a i n e d d u r i n g t r i a l s elsewhere i n t h e P a c i f i c . However, t h e a n a l y s i s of l e n g t h d a t a f o r t h e m a j o r i n d i v i d u a l s p e c i e s s t o c k s r e v e a l s t h a t , w i t h t h e e x c e p t i o n of P . f l a v i p i n n i s and E > , c a r b u n c u l u s , t h e y a r e being s l i g h t l y u n d e r -f i s h e d r a t h e r than o v e r - -f i s h e d .
P o s s i b l y t h e e s t i m a t e of MSY for t h e Marianas i s c o n s e r v a t i v e f o r Tonga. As t h e programme c o n t i n u e s , we hope t o a p p r o a c h t h e t r u e MSY f o r Tonga w i t h g r e a t e r a c c u r a c y . New f i s h i n g grounds r e c e n t l y d i s c o v e r e d w i l l be t h e s i t e of d e p l e t i o n e x p e r i m e n t s , a c r u c i a l s t e p towards e s t i m a t i n g MSY b a s e d on d a t a from Tonga r a t h e r than e l s e w h e r e . F u r t h e r b a t h y m e t r i e work w i l l a s s i s t in more a c c u r a t e mapping of f i s h i n g g r o u n d s .
6. ACKNOWLEDGEMENTS
We wish e s p e c i a l l y t o thank Dr J . P o l o v i n a who s u p e r v i s e d our d a t a a n a l y s i s , g i v i n g us much of h i s v a l u a b l e time and p a t i e n t t u i t i o n . Thanks a l s o t o Ron Benco, h i s computer programmer. We thank Dr. R. Shomura, D i r e c t o r , S . W . F . C , Honolulu l a b o r a t o r y , who made the w h o l e v i s i t p o s s i b l e . We a l s o t h a n k Dr. J . M u n r o , T . P o l a c h e c k , C . W a l t e r s , R . H i l b o r n f o r t h e i r c r i t i c i s i m s and s u g g e s t i o n s .
In Tonga, we thank a l l r e s e a r c h s t a f f who a s s i s t e d in d a t a c o l l e c t i o n , P . Mead for h i s a d v i c e , and t h e fishermen f o r t h e i r p a t i e n t c o o p e r a t i o n .
We acknowledge w i t h g r a t i t u d e t h e funds provided by ADAB t h r o u g h FFA and t h e FAO/UNDP funds which made t h e data a n a l y s i s p o s s i b l e .
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