- Email: [email protected]
Norway lobster (Nephrops norvegicus) from the southwest and south of Portugal — estimation of the effects of changing trawl mesh size and fishing effort by length cohort analysis
Fisheries Research, 17 ( 1 9 9 3 ) 2 5 9 - 2 7 1
259
0165-7836/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Norway lobster (Nephrops norvegicus) from the southwest and south of Portugal estimation of the effects of changing trawl mesh size and fishing effort by length cohort analysis F~tima Cardador lnstituto Nacional de lnvestiga¢~o das Pescas (1NIP), Av. Brasilia, 1400 Lisbon, Portugal (Accepted 9 March 1993)
Abstract
Nephrops stocks from the southwest and south of Portugal (Alentejo and Algarve) have been assessed to investigate the effects produced by increasing trawl mesh size and varying fishing effort. A length-based assessment method was applied, by sex, and the input data and parameters were those used by the 1991 International Council for the Exploration of the Sea Working Group on Nephrops Stocks. Five different trawl mesh sizes (60, 65, 70, 75 and 80 mm) and fishing effort changes from - 90% to + 50% relative to the current level of effort were simulated. Current trawl mesh size is equal to 55 mm, which is the minimum legal size for trawl nets directed to catch Nephrops. The main results from this study indicate benefits in long-term yield and biomass when trawl mesh size is increased; benefits for males are greater than those for females. If only trawl mesh size increases, long-term landings benefits range from 4 to 18% and from 2 to 9% for males and females, respectively. If reductions in fishing effort occur simultaneously with increases in trawl mesh size, the estimated landings and stock biomass benefits are higher than those obtained at current level of effort.
Introduction According to the Working Group on Nephrops Stocks (ICES, 1989), Norway lobster distributed in the southwest (Alentejo) and south (Algarve) of Portuguese waters (Fig. 1 ) comprises two stocks. Nephropsin these areas were caught by Spanish and Portuguese trawl fleets up to 1982. Since 1983 the Spanish trawlers have not been allowed to catch in Portuguese waters. Portuguese landings are carded out by trawlers and creelers. Landings from creels have been reported since 1987, but they represent only 10% of the total landings. Total landings of Nephrops reached a maximum in 1978 (2100 t) and then decreased progressively until the end of the Spanish fishery in 1982. Total landings increased again, from 830 t in 1986 to 1500 t in 1987, but the officially reported landings declined sharply in 1989 to 385 t. The 1990 reported
260
~: Cardador / Fisheries Research 17 (1993) 259-271
39~0
38°5
38~0
ines
37~5
Portim6o
37~0
38 o5
I
1]o 0
10°.0
I
9°0
I\'.",';':';'.",~.'~q0m
8°0
7°0
Fig. 1. Distribution of Nephropsnorvegicusstocks from the southwest and south of Portugal (Alentejo and Algarve). total landings of 650 t represent a substantial increase of 68%, when compared with the 1989 landings. Several studies to evaluate the effects on these Nephrops stocks of increasing trawl mesh size have been carried out. The most recent results were published by Cardador and Caramelo (1989) and in the 1990 ICES Nephrops Working Group Report (ICES, 1990); the first study concerns the effects of increasing trawl mesh size to 65 and 80 m m and the last one refers to 65 m m only. During the 1991 ICES Working Group on NephropsStocks (ICES, 1991 ) these stocks were assessed in order to evaluate the effects on the stock biomass and landings caused by changing the fishing effort. The intent of this study is to provide immediate, m e d i u m and long-term effects by increasing the trawl mesh size in combination with changes in fishing effort. The method applied is length-based cohort analysis (LCA) (Jones, 1984) and the data used are the same as those adopted by the ICES 1991 Working Group on Nephrops Stocks (ICES, 1991 ).
F. Cardador / Fisheries Research 17 (1993) 259-271
261
Input data and parameters
Length composition data The average length compositions of Nephrops landings during 1984-1990 were considered as the reference period to perform length cohort analysis (LCA). However, instead of using absolute frequencies by length group it was decided to use the percentages, because the official figures for 1986-1990 landings are not considered reliable and, at present, there is no available basis to estimate the catches. Therefore, the derived results are expressed in relative terms. LCA was performed by sex in 2 m m carapace length groups.
Growth data Table 1 presents the input parameters for males and females used in length composition analysis. Growth parameters by sex, L~ and K, and natural mortality, M, have been estimated at 1990 Working Group on Nephrops Stocks (ICES, 1990). For females, two growth curves were considered (immature and mature) because it is known that growth changes after maturation, being 26 m m the carapace length corresponding to the size at 50% maturity (ICES, 1990). The weight/length relationship used was W = 0.00028 t 32229 for males and W=0.00056 L 3-°288 for females, where Wis the total body weight in g and L is the carapace length in m m (Caramelo, 1988 ). Fishing mortality for the plus group (90% of Loo ) was set at 0.3 for males and 0.1 for females (ICES, 1991 ). Table 1 Input parameters used in length cohort analysis for male and female Nephrops Parameters
Males
L~ (mm) L~ immature (mm) Loo m a t u r e ( m m ) K (year - I ) K ( y e a r - ~) i m m a t u r e K ( y e a r - ~) mature M (year - l ) M ( y e a r - ~) i m m a t u r e M ( y e a r - ~) mature
70
Females
70 65 0.20 0.20 0.068 0.20 0.20 0.10
Loo and K are von Bertalanffy (Bertalanffy, 1938 ) growth parameters and M is the instantaneous loss due to natural causes.
262
F. Cardador / Fisheries Research 17 (1993) 259-271
Selectivity data Trawl selectivity data concerning selection factor (SF) and selection ratio (fl) values are as follows (Caramelo, 1988) Selection factor = 0.46
fl= ( L 7 5 % - L25% )/L50% = 0.50
where L75%, L25% and L50% are the lengths corresponding to 75%, 25% and 50% of retention. Current trawl mesh size is 55 mm, which is the legal minimum mesh size for trawl nets directed to catch Nephrops. Table 2 Output from length cohort analysis for male Norway lobster (SW and S of Portugal ) Size L (mm)
18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64
Removalsa (%)
0.1 0.3 0.8 4.4 7.9 12.1 10.5 9.7 10.3 8.4 7.1 6.9 5.4 5.5 3.1 2.3 1.6 1.2 1.2 0.6 0.4 0.1 0.1 0.0
dt b
0.1961 0.2041 0.2128 0.2223 0.2326 0.2440 0.2565 0.2703 0.2858 0.3031 0.3227 0.3450 0.3705 0.4002 0.4351 0.4766 0.5268 0.5889 0.6677 0.7708 0.9116 1.1157 1.4384
Fc
0.0017 0.0092 0.0258 0.1438 0.2680 0.4478 0.4338 0.4521 0.5515 0.5268 0.5191 0.6124 0.5966 0.7944 0.6077 0.5757 0.5210 0.5405 0.7775 0.6374 0.7487 0.5371 0.3980 0.3000
Note: % length distribution used; numbers and biomass are relative. L, carapace length. aLandings in %. t"Fhe time taken to grow from one length to the next. CAnnual fishing mortality (F) at length (carapace length in mm ).
Population Numbers
Biomass
164.4 158.0 151.4 144.3 133.7 119.9 102.4 87.0 73.0 58.9 47.2 37.4 28.3 21.1 14.1 10.0 6.9 4.7 3.0 1.6 0.8 0.3 0.2 0.1
117.0 161.4 215.6 276.8 338.5 391.4 434.6 473.1 497.5 507.8 511.0 497.3 467.3 414.4 355.8 311.0 268.9 226.8 169.7 114.8 72.3 42.3 26.1 28.9
F. Cardador / Fisheries Research 17 (1993) 259-271
263
Method Length cohort analysis (LCA) (Jones, 1984) was performed for each sex using the input data and parameters mentioned above. Data from LCA are presented in Tables 2 and 3, for males and females, respectively. Simulations for changing mesh size and fishing effort were undertaken using software prepared and kindly offered by N. Bailey of the Aberdeen
Laboratory. Five increments in trawl mesh size were evaluated, i.e. 60, 65, 70, 75 and 80 mm. Variations in fishing effort were set from - 90% to + 50% in relation to current level of fishing effort.
Table 3 Output from length cohort analysis for female Norway lobster (SW and S of Portugal ) Size L (mm)
18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60
Removals a (%)
0.1 0.6 1.6 6.5 11.4 13.8 12.9 15.5 13.9 8.3 6.9 3.4 2.4 1.4 0.6 0.4 0.1 0.1 0.0 0.1 0.0 0.0
dt b
0.1961 0.2041 0.2128 0.2223 0.2326 0.8172 0.8653 0.9194 0.9808 1.0509 1.1318 1.2262 1.3378 1.4718 1.6357 1.8406 2.1044 2.4567 2.9510 3.6958 4.9481
Fc
0.0032 0.019 ! 0.0544 0.2206 0.4156 0.1686 0.1884 0.2890 0.3626 0.3179 0.4028 0.3116 0.3472 0.3355 0.2495 0.2507 0.1137 0.0955 0.0655 0.1456 0.1121 0.1000
Note: % length distribution used; numbers and biomass are relative. L, carapace length. ~Landings in %. ~Fhe time taken to grow from one length to the next. CAnnual fishing mortality ( F ) at length (carapace length in m m ).
Population Numbers
Biomass
158.5 152.3 145.6 138.0 125.6 111.4 89.5 69.7 48.8 31.0 20.0 11.3 6.8 3.8 2.0 1.1 0.6 0.4 0.2 0.1 0.1 0.0
127.4 172.1 225.0 281.0 333.8 1230.5 1262.0 1199.4 1022.4 829.0 636.0 467.4 340.8 232.1 159.9 111.5 82.5 68.0 56.3 40.1 22.6 25.0
264
F. Cardador / Fisheries Research 17 (I 993) 259-271
Results
Effects on Nephrops males For males, the percentage changes in landings and in stock biomass derived from simultaneous increases in trawl mesh size and variations in fishing effort are given in Table 4. Figure 2 presents the long-term variations in landings and in biomass due to changes in fishing effort, for the current 55 m m trawl mesh size. The long-term yield and biomass curves indicate that the current level of fishing effort is 50% above the level corresponding to Fmax. A reduction in current F to Fma x would improve long-term landings by 13% and biomass by 117%, assuming no change in mesh size. The mean landing size will increase from 36.0 mm to 39.7 m m (carapace length). Figure 3 illustrates the long-term changes in landings (in % ) resulting from increasing trawl mesh size to 65 and 80 mm and for different relative levels of fishing effort. The results indicate that, at the current level of fishing effort, long-term yields increase by 4%, 8%, 12%, 15% and 18%, respectively, for 60, 65, 70, 75 and 80 mm mesh sizes. The mean landing size will increase from 36.0 m m to 36.7, 37.5, 38.3, 39.1 and 39.9 mm, depending on the progressive increase in mesh size. Table 5 summarizes the changes in the landings of males, from the first to the third year, as well as the long-term changes in landings and in stock biomass. The results are expressed in terms of percentages relative to the current landings and biomass. The estimated immediate losses (first year) in the landings are 2%, 5%, 8%, 12% and 16%. However, after the second year, increases will occur with all mesh sizes. The results presented in Table 4 indicate that, if trawl mesh size increases to 60 or 65 mm, maximum long-term yields are achieved by a 50% decrease in the current F level; if mesh size increases to 70, 75 or 80 mm, Fmax corresponds to a 40% decrease in the current fishing effort. Long-term yields at Fmax increase by 16%, 18%, 20%, 22% and 23% and stock biomass by 130%, 144%, 118%, 135% and 153%, for progressive increases in mesh size.
Effects on Nephrops females Long-term changes in yields and in stock biomass, expressed in terms of percentages, are indicated in Table 6. Figure 4 shows the long-term effects on landings and stock biomass for different levels of fishing effort if current trawl mesh size is maintained. Maximum yield is obtained with a 30-40% reduction in fishing effort; the derived benefits are 3% in landings and 48-76% in stock biomass. The mean landing size will increase from 32.6 mm to 34.034.5 mm.
-36.9 -6.2 7.6 12.7 13.3 11.8 9.1 6.1 3.0 0.0 -2.9 -5.6 -8.1 -10.5 -12.7
631.4 404.2 264.5 175.8 117.3 77.4 49.0 28.2 12.3 0.0 -9.9 -17.9 -24.6 -30.2 -35.0
-37.4 -6.1 8.5 14.3 15.6 14.5 12.3 9.6 6.7 3.8 1.0 -1.6 -4.1 -6.4 -8.6
(%) 641.6 417.7 278.5 189.2 129.9 89.0 59.8 38.2 21.7 8.8 - 1.6 -10.1 -17.1 -23.1 -28.2
(%)
Biomass
Landings
(%)
Biomass
Landings
(%)
Mesh size 60 mm
Mesh size 55 mm
Average reference year 1984-1990. Current mesh size is 55 mm.
-90.0 -80.0 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 - 10.0 0.0 10.0 20.0 30.0 40.0 50.0
Effort change
-38.1 -6.4 9.0 15.7 17.7 17.2 15.4 13.0 10.4 7.7 5.0 2.5 0.1 -2.2 -4.3
(%)
Landings
653.4 433.5 294.9 204.9 144.5 102.5 72.3 49.8 32.6 19.0 8.1 -0.9 -8.5 -14.9 -20.4
(%)
Biomass
Mesh size 65 mm
-39.2 -7.1 9.2 16.8 19.5 19.6 18.3 16.3 14.0 11.5 9.0 6.6 4.3 2.1 0.0
(%)
Landings
666.5 451.0 313.2 222.5 160.9 117.7 86.4 62.9 44.8 30.5 18.9 9.3 1.3 -5.6 -11.5
(%)
Biomass
Mesh size 70 mm
-40.6 -8.2 9.0 17.4 21.0 21.8 21.0 19.4 17.4 15.2 12.9 10.7 8.5 6.4 4.3
(%)
Landings
680.7 470.3 333.4 242.0 179.2 134.6 102.0 77.5 58.4 43.3 30.9 20.7 12.1 4.7 - 1.7
(%)
Biomass
Mesh size 75 mm
Table 4 Long-term effects (in %) by changing effort and trawl mesh size on male Norway lobster (SW and S of Portugal )
-42.3 -9.8 8.1 17.5 21.9 23.4 23.2 22.1 20.4 18.5 16.5 14.4 12.4 10.4 8.4
(%)
Landings
696.3 491.5 355.8 263.7 199.4 153.4 119.3 93.5 73.4 57.3 44.1 33.2 23.9 16.0 9.1
(%)
Biomass
Mesh size 80 m m
t,9 O~
7n
266
F. CardadorI FisheriesResearch 17 (1993) 259-271 50
650
z,O.
"
1550
L~
30"
zS z
20-
i/,50
z
10-
-350 m
_
z
<%
< -iso
-- -20 -30
.50
-40
-9'o-do
"~o-;o-~o-4-3'o-2o
,
/o
;
r
io 20 ;o ~o so
-50
EFFORT CHANGE (°/,)
Fig. 2. Long-term effects on landings and biomass (in %) due to changes in fishing effort, with current trawl mesh size (55 ram) for male Nephrops norvegicus (SW and S of Portugal). 45
t 35, ~ z
25
z
15.
--'#- 65ram
-o-- 8 0 mm
S
~
z_ 5~ -5.
z < I
~
-T5-25-35-45
-90' -8 ' o - ; o
~ ~'-~'o 0 -40~ -30' -2'o -10' '
0
' 1'o 20
' 30
' 40
50
EFFORT CHANGE (*/*)
Fig. 3. Long-term effects on landings (in %) due to changes in fishing effort and increasing trawl mesh size from 55 mm to 65 and 80 mm in male Nephrops norvegicus (SW and S of Portugal). Table 5 Short, intermediate and long-term effects in the landings and long-term effects in the biomass (%) relative to the current situation in male Norway lobster (SW and S of Portugal ) Mesh size (mm)
First year
Second year
Third year
Long-term landings
Long-term biomass
60 65 70 75 80
-2.2 -4.9 -8.2 - 12.1 - 16.3
1.1 1.8 2.1 1.7 0.6
2.8 5.6 8.0 10.0 11.2
3.8 7.7 11.5 15.2 18.2
8.8 19.0 30.5 43.3 57.3
Average reference year 1984-1990. Current mesh size is 55 mm.
565.6 374.9 251.5 169.8 114.4 75.7 48.0 27.5 12.0 0.0 -9.6 -17.4 -23.9 -29.3 -34.0
580.1 394.7 272.2 189.6 132.6 92.3 63.1 41.3 24.7 11.7 1.3 -7.2 -14.3 -20.2 -25.3
-55.0 -28.1 - 12.2 -3.0 2.1 4.6 5.7 5.7 5.3 4.5 3.5 2.4 1.2 0.1 -1.0
-53.1 -26.1 - 10.7 -2.2 2.1 4.1 4.6 4.2 3.4 2.3 1.I -0.1 -1.4 -2.6 -3.8
-51.6 -24.5 -9.7 -2.0 1.7 3.1 3.1 2.4 1.3 0.0 - 1.4 -2.8 -4.1 -5.4 -6.7
-90.0 -80.0 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 - 10.0 0.0 10.0 20.0 30.0 40.0 50.0
596.5 417.3 296.2 212.8 154.1 111.9 80.9 57.5 39.5 25.4 14.0 4.7 -3.1 -9.7 -15.3
Biomass (%)
Landings (%)
Landings (%)
Biomass (%)
Mesh size 65 mm
Mesh size 60 mm
Effort Mesh size 55 m m change Landings Biomass (%) (%) -57.2 -30.7 - 14.3 -4.4 1.4 4.7 6.3 6.9 6.8 6.3 5.5 4.6 3.7 2.7 1.6
Landings (%) 613.9 441.8 322.6 238.5 178.2 134.1 101.1 76.0 56.5 41.0 28.4 18.1 9.5 2.2 -4.0
Biomass (%)
Mesh size 70 mm
-59.6 -33.7 - 17.0 -6.5 0.1 4.1 6.3 7.4 7.8 7.7 7.2 6.6 5.8 4.9 4.1
Landings (%) 632.0 467.7 350.9 266.7 204.9 158.8 123.8 96.8 75.6 58.6 44.7 33.3 23.7 15.6 8.6
Biomass (%)
Mesh size 75 mm
Table 6 Long-term effects (%) by changing effort and trawl mesh size in female Norway lobster (SW and S of Portugal )
-62.2 -37.0 -20.3 -9.2 - 1.9 2.8 5.7 7.4 8.2 8.5 8.4 8.0 7.5 6.8 6.1
Landings (%)
650.2 494.4 380.7 296.7 233.7 185.7 148.4 119.8 96.7 78.1 62.8 50.1 39.4 30.3 22.4
Biomass (%)
Mesh size 80 m m
O~
,p
3
268
F. Cardador / Fisheries Research 17 (1993) 259-271 :0
. 650
/
-o- Biomass
---~- Landing s
30-
( FSSO
2o.
z--
10-
450
<
o 350
z
-10 z
z --
-20-
250
< :I:
o
v~ ,,~
z
< I
o
-30 -Z, 0 ~ ,50
-50"
-60
' -90
-
8~ 0
. -70
. . . . -50 -50 -40
-30
EFFORT
2~ - 0
-1
CHANGE
' 0 ' I ' 0 ' 0
20
' 30
2
0
50
-50
('/,)
Fig. 4. Long-term effects on landings and biomass (in %) due to changes in fishing effort, with current trawl mesh size (55 ram) in female Nephrops norvegicus (SW and S of Portugal). 40 65mm
-o.- 80ram
3020
z~
~0
z <
0
z_ - 1 0 . ~ o
-20
z
~ o
-30
~
-40 -50 -60 70
-~'o-8'0 Wo-~'o-so-,-;-~o-~'o-,o EFFORT
CHANGE
i
J
o
T
~'o ~o 3'0 20 50
(*/*)
Fig. 5. Long-term effects on landings (in %) due to changes in fishing effort and increasing trawl mesh size from 55 mm to 65 and 80 mm in female Nephropsnorvegicus(SW and S of Portugal).
Figure 5 illustrates the long-term effects on landings (in %) with increasing mesh size to 65 and 80 mm and for different relative levels of fishing effort. At the current level of effort, long-term yields increase by 2%, 5%, 6%, 8% and 9% if trawl mesh size increases to 60 mm, 65 mm, 70 mm, 75 mm and 80 mm, respectively. Table 7 summarizes the changes in landings of females, from the first to the fourth year, as well as the long-term changes in stock
F. Cardador / Fisheries Research 17 (1993) 259-271
269
Table 7 Short and intermediate effects in the landings and long-term effects in the biomass (%) relative to the current situation in female Norway lobster (SW and S of Portugal) Mesh size (mm)
First year
Second year
Third year
Fourth year
Long-term biomass
60 65 70 75 80
-6.0 -12.6 -19.4 -26.2 -32.8
-2.2 -5.0 -8.5 - 12.6 - 17.2
0.5 0.5 -0.2 - 1.7 -4.2
2.0 3.8 5.1 5.7 5.4
11.7 25.4 41.0 58.6 78.1
biomass. The results are expressed in terms of percentages relative to the current landings and biomass. Immediate losses in landings vary from 6% to 33% depending on the progressive increase in mesh size. For females, after the second year, there are no losses with 60 and 65 m m meshes, but for the other mesh sizes the improvement in the landings starts in the fourth year after the change. The mean landing size of females will increase from 32.6 m m to 33.2 m m , 33.9 m m , 34.5 m m , 35.1 m m and 35.7 m m if trawl mesh size increases from 55 m m to 60 m m , 65 m m , 70 m m , 75 m m and 80 mm, respectively. The results shown in Table 6 indicate that, if increase in mesh size is associated with a change in fishing effort, m a x i m u m yield may be achieved with a 30% decrease in the F l e v e l for 60 m m mesh, a 20% decrease for 65 and 70 m m and a 10% decrease for 75 m m . In the case of 80 m m mesh, the Fmax level corresponds to the current level of fishing effort (Fig. 5 ). Long-term yields at Fmax increase by 5%, 6%, 7%, 8% and 9% and stock biomass by 63%, 57%, 76%, 76% and 78% for progressive increases in mesh size. Discussion and conclusions
The results of the simulations performed show that if recruitment remains at average levels, the landings of Nephropsstocks from the southwest and south of Portugal will improve if increases in trawl mesh size take place. Those increases are most important for males, which are more heavily exploited than females. In the case of Nephrops males, the estimated increases range from 4% to 18% and from 9% to 57% for yield and stock biomass, respectively, depending on the mesh size used if the current level of fishing effort is maintained. The short-term losses can be compensated for 2 years after the increase in trawl mesh size. The predictions for Nephrops females also indicate improvement in longterm yield and biomass if increases in trawl mesh size occur, however those benefits are lower than for males.
270
F. Cardador / Fisheries Research 17 (1993) 259-271
For both sexes of Nephrops, increases in trawl mesh size and simultaneous reductions in fishing effort will provide greater long-term benefits in yield and stock biomass. Those results should be considered with some limitations considering that the method applied, i.e. length cohort analysis assumes steady state conditions and consequently the estimates of long-term yield and biomass may be accepted only if recruitment remains at average levels. As examined by Addison and Bennett ( 1992 ), for females of the crab ( Cangerpagurus L. ) in the Yorkshire fishery, long-term yield produced by increasing minimum landing size depends upon the nature of the stock-recruitment relationship. They estimated, for a particular range of increases in minimum landing size, that if recruitment increases the predicted yield derived from LCA is underestimated and if recruitment decreases the benefits will be less than the predicted yield. Comparing the effects estimated in the present study with those published in the 1990 Working Group Report on Nephrops Stocks (ICES, 1990), by increasing trawl mesh size to 65 mm, the benefits in yield and biomass obtained for both sexes are slightly lower than the benefits estimated by the Working Group. In ICES (1990), the increases in yield and biomass for males are 12% and 29%, and for females 6% and 38%, respectively. The present study indicates benefits around 8% and 19% for males and 5% and 25% for females, in long-term yield and biomass, respectively. These differences must be mainly due to the adopted current trawl mesh size, which in the case of ICES (1990) is 50 m m and in this study is 55 mm. Although in quantitative terms there are slight differences, the results provided from both studies indicate benefits if the trawl mesh size increases.
References Addison,J.T. and Bennett,D.B., 1992. Assessmentof minimumlandingsizesof the ediblecrab, CancerpagurusL., on the east coast of England. Fish. Res., 13: 67-88. Caramelo, A.M., 1988. Effectsof a changein meshsize on Nephropsstocksin Portuguesewaters. Fish. Res., 6: 379-391. Cardador, F. and Caramelo,A.M., 1989. Effectsof managementstrategieson the catchof hake, horse mackerel, bluewhiting and nephrops in ICES Divisions VIIIc and IXa.ICES Doc. C.M.1989/H:39, 24 pp. (mimeo). ICES, 1989. Report of the WorkingGroup on NephropsStocks.Lowestoft,England, 10-14 April 1989. ICES Doc. C.M.1989/Assess:18, 163 pp. ICES, 1990. Report of the WorkingGroup on NephropsStocks. Nantes, France, 21-28 March 1990. ICESDoc. C.M.1990/Assess:16, 172 pp. ICES, 1991. Reportof the WorkingGroup on NephropsStocks.Aberdeen,28 February-6March 1991. ICESDoc. C.M.1991/Assess:l 1,197 pp.
F. Cardador / Fisheries Research 17 (1993) 259- 271
27 1
Jones, R., 1984. Assessing the effects of changes in exploitation pattern using length composition data (with some notes on VPA and cohort analysis). FAO Fish. Tech. Pap., 256, 117 PP. Von Bertalanffy, L., 1938. A quantitative theory of organic growth. Human Biol., l 0 (2)" 181 213.
259
0165-7836/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Norway lobster (Nephrops norvegicus) from the southwest and south of Portugal estimation of the effects of changing trawl mesh size and fishing effort by length cohort analysis F~tima Cardador lnstituto Nacional de lnvestiga¢~o das Pescas (1NIP), Av. Brasilia, 1400 Lisbon, Portugal (Accepted 9 March 1993)
Abstract
Nephrops stocks from the southwest and south of Portugal (Alentejo and Algarve) have been assessed to investigate the effects produced by increasing trawl mesh size and varying fishing effort. A length-based assessment method was applied, by sex, and the input data and parameters were those used by the 1991 International Council for the Exploration of the Sea Working Group on Nephrops Stocks. Five different trawl mesh sizes (60, 65, 70, 75 and 80 mm) and fishing effort changes from - 90% to + 50% relative to the current level of effort were simulated. Current trawl mesh size is equal to 55 mm, which is the minimum legal size for trawl nets directed to catch Nephrops. The main results from this study indicate benefits in long-term yield and biomass when trawl mesh size is increased; benefits for males are greater than those for females. If only trawl mesh size increases, long-term landings benefits range from 4 to 18% and from 2 to 9% for males and females, respectively. If reductions in fishing effort occur simultaneously with increases in trawl mesh size, the estimated landings and stock biomass benefits are higher than those obtained at current level of effort.
Introduction According to the Working Group on Nephrops Stocks (ICES, 1989), Norway lobster distributed in the southwest (Alentejo) and south (Algarve) of Portuguese waters (Fig. 1 ) comprises two stocks. Nephropsin these areas were caught by Spanish and Portuguese trawl fleets up to 1982. Since 1983 the Spanish trawlers have not been allowed to catch in Portuguese waters. Portuguese landings are carded out by trawlers and creelers. Landings from creels have been reported since 1987, but they represent only 10% of the total landings. Total landings of Nephrops reached a maximum in 1978 (2100 t) and then decreased progressively until the end of the Spanish fishery in 1982. Total landings increased again, from 830 t in 1986 to 1500 t in 1987, but the officially reported landings declined sharply in 1989 to 385 t. The 1990 reported
260
~: Cardador / Fisheries Research 17 (1993) 259-271
39~0
38°5
38~0
ines
37~5
Portim6o
37~0
38 o5
I
1]o 0
10°.0
I
9°0
I\'.",';':';'.",~.'~q0m
8°0
7°0
Fig. 1. Distribution of Nephropsnorvegicusstocks from the southwest and south of Portugal (Alentejo and Algarve). total landings of 650 t represent a substantial increase of 68%, when compared with the 1989 landings. Several studies to evaluate the effects on these Nephrops stocks of increasing trawl mesh size have been carried out. The most recent results were published by Cardador and Caramelo (1989) and in the 1990 ICES Nephrops Working Group Report (ICES, 1990); the first study concerns the effects of increasing trawl mesh size to 65 and 80 m m and the last one refers to 65 m m only. During the 1991 ICES Working Group on NephropsStocks (ICES, 1991 ) these stocks were assessed in order to evaluate the effects on the stock biomass and landings caused by changing the fishing effort. The intent of this study is to provide immediate, m e d i u m and long-term effects by increasing the trawl mesh size in combination with changes in fishing effort. The method applied is length-based cohort analysis (LCA) (Jones, 1984) and the data used are the same as those adopted by the ICES 1991 Working Group on Nephrops Stocks (ICES, 1991 ).
F. Cardador / Fisheries Research 17 (1993) 259-271
261
Input data and parameters
Length composition data The average length compositions of Nephrops landings during 1984-1990 were considered as the reference period to perform length cohort analysis (LCA). However, instead of using absolute frequencies by length group it was decided to use the percentages, because the official figures for 1986-1990 landings are not considered reliable and, at present, there is no available basis to estimate the catches. Therefore, the derived results are expressed in relative terms. LCA was performed by sex in 2 m m carapace length groups.
Growth data Table 1 presents the input parameters for males and females used in length composition analysis. Growth parameters by sex, L~ and K, and natural mortality, M, have been estimated at 1990 Working Group on Nephrops Stocks (ICES, 1990). For females, two growth curves were considered (immature and mature) because it is known that growth changes after maturation, being 26 m m the carapace length corresponding to the size at 50% maturity (ICES, 1990). The weight/length relationship used was W = 0.00028 t 32229 for males and W=0.00056 L 3-°288 for females, where Wis the total body weight in g and L is the carapace length in m m (Caramelo, 1988 ). Fishing mortality for the plus group (90% of Loo ) was set at 0.3 for males and 0.1 for females (ICES, 1991 ). Table 1 Input parameters used in length cohort analysis for male and female Nephrops Parameters
Males
L~ (mm) L~ immature (mm) Loo m a t u r e ( m m ) K (year - I ) K ( y e a r - ~) i m m a t u r e K ( y e a r - ~) mature M (year - l ) M ( y e a r - ~) i m m a t u r e M ( y e a r - ~) mature
70
Females
70 65 0.20 0.20 0.068 0.20 0.20 0.10
Loo and K are von Bertalanffy (Bertalanffy, 1938 ) growth parameters and M is the instantaneous loss due to natural causes.
262
F. Cardador / Fisheries Research 17 (1993) 259-271
Selectivity data Trawl selectivity data concerning selection factor (SF) and selection ratio (fl) values are as follows (Caramelo, 1988) Selection factor = 0.46
fl= ( L 7 5 % - L25% )/L50% = 0.50
where L75%, L25% and L50% are the lengths corresponding to 75%, 25% and 50% of retention. Current trawl mesh size is 55 mm, which is the legal minimum mesh size for trawl nets directed to catch Nephrops. Table 2 Output from length cohort analysis for male Norway lobster (SW and S of Portugal ) Size L (mm)
18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64
Removalsa (%)
0.1 0.3 0.8 4.4 7.9 12.1 10.5 9.7 10.3 8.4 7.1 6.9 5.4 5.5 3.1 2.3 1.6 1.2 1.2 0.6 0.4 0.1 0.1 0.0
dt b
0.1961 0.2041 0.2128 0.2223 0.2326 0.2440 0.2565 0.2703 0.2858 0.3031 0.3227 0.3450 0.3705 0.4002 0.4351 0.4766 0.5268 0.5889 0.6677 0.7708 0.9116 1.1157 1.4384
Fc
0.0017 0.0092 0.0258 0.1438 0.2680 0.4478 0.4338 0.4521 0.5515 0.5268 0.5191 0.6124 0.5966 0.7944 0.6077 0.5757 0.5210 0.5405 0.7775 0.6374 0.7487 0.5371 0.3980 0.3000
Note: % length distribution used; numbers and biomass are relative. L, carapace length. aLandings in %. t"Fhe time taken to grow from one length to the next. CAnnual fishing mortality (F) at length (carapace length in mm ).
Population Numbers
Biomass
164.4 158.0 151.4 144.3 133.7 119.9 102.4 87.0 73.0 58.9 47.2 37.4 28.3 21.1 14.1 10.0 6.9 4.7 3.0 1.6 0.8 0.3 0.2 0.1
117.0 161.4 215.6 276.8 338.5 391.4 434.6 473.1 497.5 507.8 511.0 497.3 467.3 414.4 355.8 311.0 268.9 226.8 169.7 114.8 72.3 42.3 26.1 28.9
F. Cardador / Fisheries Research 17 (1993) 259-271
263
Method Length cohort analysis (LCA) (Jones, 1984) was performed for each sex using the input data and parameters mentioned above. Data from LCA are presented in Tables 2 and 3, for males and females, respectively. Simulations for changing mesh size and fishing effort were undertaken using software prepared and kindly offered by N. Bailey of the Aberdeen
Laboratory. Five increments in trawl mesh size were evaluated, i.e. 60, 65, 70, 75 and 80 mm. Variations in fishing effort were set from - 90% to + 50% in relation to current level of fishing effort.
Table 3 Output from length cohort analysis for female Norway lobster (SW and S of Portugal ) Size L (mm)
18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60
Removals a (%)
0.1 0.6 1.6 6.5 11.4 13.8 12.9 15.5 13.9 8.3 6.9 3.4 2.4 1.4 0.6 0.4 0.1 0.1 0.0 0.1 0.0 0.0
dt b
0.1961 0.2041 0.2128 0.2223 0.2326 0.8172 0.8653 0.9194 0.9808 1.0509 1.1318 1.2262 1.3378 1.4718 1.6357 1.8406 2.1044 2.4567 2.9510 3.6958 4.9481
Fc
0.0032 0.019 ! 0.0544 0.2206 0.4156 0.1686 0.1884 0.2890 0.3626 0.3179 0.4028 0.3116 0.3472 0.3355 0.2495 0.2507 0.1137 0.0955 0.0655 0.1456 0.1121 0.1000
Note: % length distribution used; numbers and biomass are relative. L, carapace length. ~Landings in %. ~Fhe time taken to grow from one length to the next. CAnnual fishing mortality ( F ) at length (carapace length in m m ).
Population Numbers
Biomass
158.5 152.3 145.6 138.0 125.6 111.4 89.5 69.7 48.8 31.0 20.0 11.3 6.8 3.8 2.0 1.1 0.6 0.4 0.2 0.1 0.1 0.0
127.4 172.1 225.0 281.0 333.8 1230.5 1262.0 1199.4 1022.4 829.0 636.0 467.4 340.8 232.1 159.9 111.5 82.5 68.0 56.3 40.1 22.6 25.0
264
F. Cardador / Fisheries Research 17 (I 993) 259-271
Results
Effects on Nephrops males For males, the percentage changes in landings and in stock biomass derived from simultaneous increases in trawl mesh size and variations in fishing effort are given in Table 4. Figure 2 presents the long-term variations in landings and in biomass due to changes in fishing effort, for the current 55 m m trawl mesh size. The long-term yield and biomass curves indicate that the current level of fishing effort is 50% above the level corresponding to Fmax. A reduction in current F to Fma x would improve long-term landings by 13% and biomass by 117%, assuming no change in mesh size. The mean landing size will increase from 36.0 mm to 39.7 m m (carapace length). Figure 3 illustrates the long-term changes in landings (in % ) resulting from increasing trawl mesh size to 65 and 80 mm and for different relative levels of fishing effort. The results indicate that, at the current level of fishing effort, long-term yields increase by 4%, 8%, 12%, 15% and 18%, respectively, for 60, 65, 70, 75 and 80 mm mesh sizes. The mean landing size will increase from 36.0 m m to 36.7, 37.5, 38.3, 39.1 and 39.9 mm, depending on the progressive increase in mesh size. Table 5 summarizes the changes in the landings of males, from the first to the third year, as well as the long-term changes in landings and in stock biomass. The results are expressed in terms of percentages relative to the current landings and biomass. The estimated immediate losses (first year) in the landings are 2%, 5%, 8%, 12% and 16%. However, after the second year, increases will occur with all mesh sizes. The results presented in Table 4 indicate that, if trawl mesh size increases to 60 or 65 mm, maximum long-term yields are achieved by a 50% decrease in the current F level; if mesh size increases to 70, 75 or 80 mm, Fmax corresponds to a 40% decrease in the current fishing effort. Long-term yields at Fmax increase by 16%, 18%, 20%, 22% and 23% and stock biomass by 130%, 144%, 118%, 135% and 153%, for progressive increases in mesh size.
Effects on Nephrops females Long-term changes in yields and in stock biomass, expressed in terms of percentages, are indicated in Table 6. Figure 4 shows the long-term effects on landings and stock biomass for different levels of fishing effort if current trawl mesh size is maintained. Maximum yield is obtained with a 30-40% reduction in fishing effort; the derived benefits are 3% in landings and 48-76% in stock biomass. The mean landing size will increase from 32.6 mm to 34.034.5 mm.
-36.9 -6.2 7.6 12.7 13.3 11.8 9.1 6.1 3.0 0.0 -2.9 -5.6 -8.1 -10.5 -12.7
631.4 404.2 264.5 175.8 117.3 77.4 49.0 28.2 12.3 0.0 -9.9 -17.9 -24.6 -30.2 -35.0
-37.4 -6.1 8.5 14.3 15.6 14.5 12.3 9.6 6.7 3.8 1.0 -1.6 -4.1 -6.4 -8.6
(%) 641.6 417.7 278.5 189.2 129.9 89.0 59.8 38.2 21.7 8.8 - 1.6 -10.1 -17.1 -23.1 -28.2
(%)
Biomass
Landings
(%)
Biomass
Landings
(%)
Mesh size 60 mm
Mesh size 55 mm
Average reference year 1984-1990. Current mesh size is 55 mm.
-90.0 -80.0 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 - 10.0 0.0 10.0 20.0 30.0 40.0 50.0
Effort change
-38.1 -6.4 9.0 15.7 17.7 17.2 15.4 13.0 10.4 7.7 5.0 2.5 0.1 -2.2 -4.3
(%)
Landings
653.4 433.5 294.9 204.9 144.5 102.5 72.3 49.8 32.6 19.0 8.1 -0.9 -8.5 -14.9 -20.4
(%)
Biomass
Mesh size 65 mm
-39.2 -7.1 9.2 16.8 19.5 19.6 18.3 16.3 14.0 11.5 9.0 6.6 4.3 2.1 0.0
(%)
Landings
666.5 451.0 313.2 222.5 160.9 117.7 86.4 62.9 44.8 30.5 18.9 9.3 1.3 -5.6 -11.5
(%)
Biomass
Mesh size 70 mm
-40.6 -8.2 9.0 17.4 21.0 21.8 21.0 19.4 17.4 15.2 12.9 10.7 8.5 6.4 4.3
(%)
Landings
680.7 470.3 333.4 242.0 179.2 134.6 102.0 77.5 58.4 43.3 30.9 20.7 12.1 4.7 - 1.7
(%)
Biomass
Mesh size 75 mm
Table 4 Long-term effects (in %) by changing effort and trawl mesh size on male Norway lobster (SW and S of Portugal )
-42.3 -9.8 8.1 17.5 21.9 23.4 23.2 22.1 20.4 18.5 16.5 14.4 12.4 10.4 8.4
(%)
Landings
696.3 491.5 355.8 263.7 199.4 153.4 119.3 93.5 73.4 57.3 44.1 33.2 23.9 16.0 9.1
(%)
Biomass
Mesh size 80 m m
t,9 O~
7n
266
F. CardadorI FisheriesResearch 17 (1993) 259-271 50
650
z,O.
"
1550
L~
30"
zS z
20-
i/,50
z
10-
-350 m
_
z
<%
< -iso
-- -20 -30
.50
-40
-9'o-do
"~o-;o-~o-4-3'o-2o
,
/o
;
r
io 20 ;o ~o so
-50
EFFORT CHANGE (°/,)
Fig. 2. Long-term effects on landings and biomass (in %) due to changes in fishing effort, with current trawl mesh size (55 ram) for male Nephrops norvegicus (SW and S of Portugal). 45
t 35, ~ z
25
z
15.
--'#- 65ram
-o-- 8 0 mm
S
~
z_ 5~ -5.
z < I
~
-T5-25-35-45
-90' -8 ' o - ; o
~ ~'-~'o 0 -40~ -30' -2'o -10' '
0
' 1'o 20
' 30
' 40
50
EFFORT CHANGE (*/*)
Fig. 3. Long-term effects on landings (in %) due to changes in fishing effort and increasing trawl mesh size from 55 mm to 65 and 80 mm in male Nephrops norvegicus (SW and S of Portugal). Table 5 Short, intermediate and long-term effects in the landings and long-term effects in the biomass (%) relative to the current situation in male Norway lobster (SW and S of Portugal ) Mesh size (mm)
First year
Second year
Third year
Long-term landings
Long-term biomass
60 65 70 75 80
-2.2 -4.9 -8.2 - 12.1 - 16.3
1.1 1.8 2.1 1.7 0.6
2.8 5.6 8.0 10.0 11.2
3.8 7.7 11.5 15.2 18.2
8.8 19.0 30.5 43.3 57.3
Average reference year 1984-1990. Current mesh size is 55 mm.
565.6 374.9 251.5 169.8 114.4 75.7 48.0 27.5 12.0 0.0 -9.6 -17.4 -23.9 -29.3 -34.0
580.1 394.7 272.2 189.6 132.6 92.3 63.1 41.3 24.7 11.7 1.3 -7.2 -14.3 -20.2 -25.3
-55.0 -28.1 - 12.2 -3.0 2.1 4.6 5.7 5.7 5.3 4.5 3.5 2.4 1.2 0.1 -1.0
-53.1 -26.1 - 10.7 -2.2 2.1 4.1 4.6 4.2 3.4 2.3 1.I -0.1 -1.4 -2.6 -3.8
-51.6 -24.5 -9.7 -2.0 1.7 3.1 3.1 2.4 1.3 0.0 - 1.4 -2.8 -4.1 -5.4 -6.7
-90.0 -80.0 -70.0 -60.0 -50.0 -40.0 -30.0 -20.0 - 10.0 0.0 10.0 20.0 30.0 40.0 50.0
596.5 417.3 296.2 212.8 154.1 111.9 80.9 57.5 39.5 25.4 14.0 4.7 -3.1 -9.7 -15.3
Biomass (%)
Landings (%)
Landings (%)
Biomass (%)
Mesh size 65 mm
Mesh size 60 mm
Effort Mesh size 55 m m change Landings Biomass (%) (%) -57.2 -30.7 - 14.3 -4.4 1.4 4.7 6.3 6.9 6.8 6.3 5.5 4.6 3.7 2.7 1.6
Landings (%) 613.9 441.8 322.6 238.5 178.2 134.1 101.1 76.0 56.5 41.0 28.4 18.1 9.5 2.2 -4.0
Biomass (%)
Mesh size 70 mm
-59.6 -33.7 - 17.0 -6.5 0.1 4.1 6.3 7.4 7.8 7.7 7.2 6.6 5.8 4.9 4.1
Landings (%) 632.0 467.7 350.9 266.7 204.9 158.8 123.8 96.8 75.6 58.6 44.7 33.3 23.7 15.6 8.6
Biomass (%)
Mesh size 75 mm
Table 6 Long-term effects (%) by changing effort and trawl mesh size in female Norway lobster (SW and S of Portugal )
-62.2 -37.0 -20.3 -9.2 - 1.9 2.8 5.7 7.4 8.2 8.5 8.4 8.0 7.5 6.8 6.1
Landings (%)
650.2 494.4 380.7 296.7 233.7 185.7 148.4 119.8 96.7 78.1 62.8 50.1 39.4 30.3 22.4
Biomass (%)
Mesh size 80 m m
O~
,p
3
268
F. Cardador / Fisheries Research 17 (1993) 259-271 :0
. 650
/
-o- Biomass
---~- Landing s
30-
( FSSO
2o.
z--
10-
450
<
o 350
z
-10 z
z --
-20-
250
< :I:
o
v~ ,,~
z
< I
o
-30 -Z, 0 ~ ,50
-50"
-60
' -90
-
8~ 0
. -70
. . . . -50 -50 -40
-30
EFFORT
2~ - 0
-1
CHANGE
' 0 ' I ' 0 ' 0
20
' 30
2
0
50
-50
('/,)
Fig. 4. Long-term effects on landings and biomass (in %) due to changes in fishing effort, with current trawl mesh size (55 ram) in female Nephrops norvegicus (SW and S of Portugal). 40 65mm
-o.- 80ram
3020
z~
~0
z <
0
z_ - 1 0 . ~ o
-20
z
~ o
-30
~
-40 -50 -60 70
-~'o-8'0 Wo-~'o-so-,-;-~o-~'o-,o EFFORT
CHANGE
i
J
o
T
~'o ~o 3'0 20 50
(*/*)
Fig. 5. Long-term effects on landings (in %) due to changes in fishing effort and increasing trawl mesh size from 55 mm to 65 and 80 mm in female Nephropsnorvegicus(SW and S of Portugal).
Figure 5 illustrates the long-term effects on landings (in %) with increasing mesh size to 65 and 80 mm and for different relative levels of fishing effort. At the current level of effort, long-term yields increase by 2%, 5%, 6%, 8% and 9% if trawl mesh size increases to 60 mm, 65 mm, 70 mm, 75 mm and 80 mm, respectively. Table 7 summarizes the changes in landings of females, from the first to the fourth year, as well as the long-term changes in stock
F. Cardador / Fisheries Research 17 (1993) 259-271
269
Table 7 Short and intermediate effects in the landings and long-term effects in the biomass (%) relative to the current situation in female Norway lobster (SW and S of Portugal) Mesh size (mm)
First year
Second year
Third year
Fourth year
Long-term biomass
60 65 70 75 80
-6.0 -12.6 -19.4 -26.2 -32.8
-2.2 -5.0 -8.5 - 12.6 - 17.2
0.5 0.5 -0.2 - 1.7 -4.2
2.0 3.8 5.1 5.7 5.4
11.7 25.4 41.0 58.6 78.1
biomass. The results are expressed in terms of percentages relative to the current landings and biomass. Immediate losses in landings vary from 6% to 33% depending on the progressive increase in mesh size. For females, after the second year, there are no losses with 60 and 65 m m meshes, but for the other mesh sizes the improvement in the landings starts in the fourth year after the change. The mean landing size of females will increase from 32.6 m m to 33.2 m m , 33.9 m m , 34.5 m m , 35.1 m m and 35.7 m m if trawl mesh size increases from 55 m m to 60 m m , 65 m m , 70 m m , 75 m m and 80 mm, respectively. The results shown in Table 6 indicate that, if increase in mesh size is associated with a change in fishing effort, m a x i m u m yield may be achieved with a 30% decrease in the F l e v e l for 60 m m mesh, a 20% decrease for 65 and 70 m m and a 10% decrease for 75 m m . In the case of 80 m m mesh, the Fmax level corresponds to the current level of fishing effort (Fig. 5 ). Long-term yields at Fmax increase by 5%, 6%, 7%, 8% and 9% and stock biomass by 63%, 57%, 76%, 76% and 78% for progressive increases in mesh size. Discussion and conclusions
The results of the simulations performed show that if recruitment remains at average levels, the landings of Nephropsstocks from the southwest and south of Portugal will improve if increases in trawl mesh size take place. Those increases are most important for males, which are more heavily exploited than females. In the case of Nephrops males, the estimated increases range from 4% to 18% and from 9% to 57% for yield and stock biomass, respectively, depending on the mesh size used if the current level of fishing effort is maintained. The short-term losses can be compensated for 2 years after the increase in trawl mesh size. The predictions for Nephrops females also indicate improvement in longterm yield and biomass if increases in trawl mesh size occur, however those benefits are lower than for males.
270
F. Cardador / Fisheries Research 17 (1993) 259-271
For both sexes of Nephrops, increases in trawl mesh size and simultaneous reductions in fishing effort will provide greater long-term benefits in yield and stock biomass. Those results should be considered with some limitations considering that the method applied, i.e. length cohort analysis assumes steady state conditions and consequently the estimates of long-term yield and biomass may be accepted only if recruitment remains at average levels. As examined by Addison and Bennett ( 1992 ), for females of the crab ( Cangerpagurus L. ) in the Yorkshire fishery, long-term yield produced by increasing minimum landing size depends upon the nature of the stock-recruitment relationship. They estimated, for a particular range of increases in minimum landing size, that if recruitment increases the predicted yield derived from LCA is underestimated and if recruitment decreases the benefits will be less than the predicted yield. Comparing the effects estimated in the present study with those published in the 1990 Working Group Report on Nephrops Stocks (ICES, 1990), by increasing trawl mesh size to 65 mm, the benefits in yield and biomass obtained for both sexes are slightly lower than the benefits estimated by the Working Group. In ICES (1990), the increases in yield and biomass for males are 12% and 29%, and for females 6% and 38%, respectively. The present study indicates benefits around 8% and 19% for males and 5% and 25% for females, in long-term yield and biomass, respectively. These differences must be mainly due to the adopted current trawl mesh size, which in the case of ICES (1990) is 50 m m and in this study is 55 mm. Although in quantitative terms there are slight differences, the results provided from both studies indicate benefits if the trawl mesh size increases.
References Addison,J.T. and Bennett,D.B., 1992. Assessmentof minimumlandingsizesof the ediblecrab, CancerpagurusL., on the east coast of England. Fish. Res., 13: 67-88. Caramelo, A.M., 1988. Effectsof a changein meshsize on Nephropsstocksin Portuguesewaters. Fish. Res., 6: 379-391. Cardador, F. and Caramelo,A.M., 1989. Effectsof managementstrategieson the catchof hake, horse mackerel, bluewhiting and nephrops in ICES Divisions VIIIc and IXa.ICES Doc. C.M.1989/H:39, 24 pp. (mimeo). ICES, 1989. Report of the WorkingGroup on NephropsStocks.Lowestoft,England, 10-14 April 1989. ICES Doc. C.M.1989/Assess:18, 163 pp. ICES, 1990. Report of the WorkingGroup on NephropsStocks. Nantes, France, 21-28 March 1990. ICESDoc. C.M.1990/Assess:16, 172 pp. ICES, 1991. Reportof the WorkingGroup on NephropsStocks.Aberdeen,28 February-6March 1991. ICESDoc. C.M.1991/Assess:l 1,197 pp.
F. Cardador / Fisheries Research 17 (1993) 259- 271
27 1
Jones, R., 1984. Assessing the effects of changes in exploitation pattern using length composition data (with some notes on VPA and cohort analysis). FAO Fish. Tech. Pap., 256, 117 PP. Von Bertalanffy, L., 1938. A quantitative theory of organic growth. Human Biol., l 0 (2)" 181 213.