The application of per-recruit models to Argyrosomus inodorus, an important South African sciaenid fish

eflSHERIES wSEARC+i ELSEVIER

Fisheries Research 30 ( 1997)103- 115

The application of per-recruit models to Argyrosomus irzodurus, an important South African sciaenid fish Marc H. Griffiths

*

Sea Fisheries Research Institute, Prioute Bag X2, Roggebuui 8012, Cape Town, South Africa Accepted 20 August

1996

Abstract

Argyrosomus inodurus is an important linefish species off the eastern seaboard of South Africa, where it comprises three discrete stocks. Per-recruit models indicate that all three stocks are exploited far beyond optimal (FsB4 and F,,,) and threshold (Fsnzs) fishing limits, and that spawner biomass-per-recruit @B/R) ratios were 2.9-12S%(SB/R),+. These results show that manual linefishing methods may deplete stocks of migratory fish, even when the juveniles are protected from exploitation. Four options for stock rebuilding, based on the results of the per-recruit analyses, are discussed in relation to the South African linefishery. It is also suggested that the existing approach for determining bag and size limits for South African linefish species does not provide sufficient protection in some cases, and that where possible control measures should be based on per-recruit analyses. 0 1997 Elsevier Science B.V. Keywords:

Argyrosomu.s inodorus; Mortality;

Yield-per-recruit;

Spawner

1. Introduction

The silver kob, Argyrosomus inodorus (Griffiths and Heemstra), is a medium-sized sciaenid (max. size 34 kg) which is known from northern Namibia on the west coast of southern Africa to the Kei River on the east coast of South Africa (Griffiths and Heemstra, 1995). Until recently it was misidentified as A. hololepidotus throughout its distribution, and off South Africa it was also confused with a sympatric species, A. juponicus (Griffiths and Heemstra, 199.5). A. inodorus have been exploited in South African waters for more than 150 years (Pappe, 1866). Combining the current market value and annual catch, the silver kob is probably the most valuable species caught by the linefishery between

l

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Ol65-7836/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO16S-7836(96)00552-8

biomass-per-recruit;

Biological

reference

points; Management

Cape Point and East London. It is not common between Cape Point and central Namibia, and on the East Coast does not occur north of the Kei River (Griffiths and Heemstra, 1995). The South African linefishery consists of about 2900 commercial (W. Kroon, Sea Fisheries, personal communication) and at least 4000 club affiliated recreational (Ferreira, 1993) vessels. These boats vary from 5 m to 15 m in length and operate in the marine environment on both east and west coasts. Fish are retrieved manually using handlines or rod and reel. A. inodorus is also landed as a by-catch of the sole and hake directed inshore (< 120 m depth) trawlfishery between Cape Agulhas and Port Alfred (Japp et al., 1994), and is caught by rock and surf anglers and commercial beach-seine fishermen between Cape Point and Cape Agulhas. Average annual catches of silver kob are presently around 835 t for the com-

104

M.H. Griffiths/Fisheries

mercial linefishery and 217 t for the inshore trawlfishery; and although estimates are not available for the recreational linefishery, the catches for this sector could be as high as their commercial counterparts (Griffiths and Heemstra, 1995). Southwestern Cape shore anglers and commercial beach-seine fishermen annually catch an estimated 26 t and 4 t of A. inodorus, respectively (Lamberth et al., 1994). Hecht and Tilney (1989) reported a substantial decline in A, inodorus (as A. hololepidotus) CPUE in the Port Alfred linefishery in the period 19821987, and also expressed concern over the large contribution of recruits to the total catch. Smale (1985) demonstrated a steady downward trend in the national trawled catch of this species (1968-1981), and also provided evidence suggesting that trawl catches in the eastern Cape had crashed between 1967 and 1972. Despite its importance to the linefishery, its lengthy period of exploitation and evidence for declining catches, no attempt has previously been made to assess the status of the South African silver kob resource, or to manage it on scientific grounds. Typical of linefisheries throughout the world (Huntsman et al., 1983; Buxton, 1992), catch and effort data for the South African fishery are diffuse and unevenly distributed in space and across user groups. In the absence of long-term catch data, and information on the spawner biomass recruit relationship, yield per-recruit (Y/R) and spawner biomass per-recruit (SB/R) models represent the most appropriate stock assessment methods available (Butterworth et al., 1989; Punt, 1993; Govender, 1995). These models incorporate the interplay between somatic growth and the probability of dying in order to predict the life-time yield of a cohort, and the spawner biomass remaining, under different combinations of fishing mortality (F) and age at first capture (t,.). Assumptions implicit in this approach are that recruitment is constant and that the stock studied is in an equilibrium state. Although these assumptions may not be realistic and consequently the absolute (vs relative) yield values may be inaccurate, these methods (on a relative basis) do allow for the quantitative assessment of the desired level of exploitation of a resource. Fisheries may be managed according to biological, sociological or economic objectives. While the

Research

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biological approach is to maximize long-term catch, socio-economic considerations (e.g. maximizing jobs), will often require higher fishing levels and corresponding lower yields. Fisheries managers therefore require estimates of harvest levels that provide maximum long-term yield as well as those at which the risk of stock depletion is unacceptably high. It has been consistently demonstrated that recruitment overfishing (and collapse) will occur in most stocks when the relative SB/R is reduced to < 20-30% of the unfished level (i.e. %(SB/R),=,)(Clark, 1991; Mace and Sissenwine, 1993; Thompson, 1993; Mace, 1994). Several target reference points (TRPs) have been suggested in attempts to define fishing mortalities, using per-recruit analyses, that will maximize longterm yield (see Clark, 1991 and Punt, 1993 for reviews). Using deterministic computations with a wide range of groundfish life histories, Clark (1991) demonstrated that a fishing mortality rate (Fs,,,) that reduces the SB/R ratio to 35%(SB/R),=, will provide high yields at low risk, regardless of the spawner-recruit relationship. Punt (1993) simulated the growth of populations initially depleted to various levels and then managed under six different strategies. By including uncertainty in population estimates, and natural variability, he showed that the FsBj5 strategy could, on occasion, reduce the spawner biomass to < 20% of unfished levels. A second study by Clark (1993) based on stochastic trials, confirmed these results, and indicated that random variation in recruitment called for a SB/R of 40%(SB/R),=,; a TRP (i.e. FsBdO) also recommended by Mace (1994) for situations where the spawner-recruit relationship is unknown. F,., (the rate of fishing mortality which corresponds to a point on the yield-per-recruit curve where the slope is 10% of that at the origin; Gulland and Boerema (1973)) has probably been more widely used than any other TRP in fisheries management (Rivard and Maguire, 1993; Hilde’n, 1993). In his simulation exercise Punt (1993) showed that the F,, , strategy provided the best combination of yield and risk (provided age-at-50% recruitment 2 age-at50%-maturity) of all the strategies he examined (including F,,, , FO,,, FSBj5 and FsssO>. In addition, F,, , generally corresponds to around FSBj5-FSB4,, when recruitment and maturity schedules coincide (Clark, 1993; Mace, 1994). The disadvantage of the

M.H.

Gri@hs/

Fisherks

F,., TRP is that it is not based on remaining spawner biomass and under certain conditions (e.g. where recruitment to the fishery occurs before sexual maturity is attained), could produce F values > FsBzO (Punt, 1993; Clark, 1993; Mace, 1994). The objectives of the present study were: (i) to determine optimal levels of F and rc for South African A. inodorus using per-recruit models; and (ii) to translate these values into practical recommendations for the management of the resource. Based on patterns of distribution, growth rates, ages at maturity, tagging data, mortality estimates and otolith morphology, Griffiths (1995) concluded that the South African silver kob occurring between Cape Point and the Kei River exist as three discrete stocks. Since population parameters varied (i.e. growth and age at maturity), per-recruit models were applied to each of these three stocks. FSBz5 was assumed to be a realistic threshold reference point and F,,, and F sB40 values were calculated as TRPs.

2. Materials

and methods

2.1. General Griffiths (1995) showed that the silver kob in the Southeastern Cape, the Southern

ring

occur-

Cape

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30 (1997)

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105

and the Southwestern Cape (Fig. 1) represented three stocks; one in each region. Stock specific data used for the per-recruit analyses of the present study included: (i) length measurements obtained from the catches of recreational and commercial linefishers (1990- 1994); (ii) age and growth data (collected 1990- 199 1) presented by Griffiths ( 1996) (see Table 1); and (iii) size and age at maturity information for female silver kob in the Southeastern Cape and in the Southern Cape (data collected 1990- 1992) described in Griffiths (1995) (see Table 1). Since size at sexual maturity has not been studied in the Southwestern Cape (Griffiths, 1995), an average of the ages at 50% female maturity in the other two regions was used. 2.2. Mortality for A. inusing two methods. Age-length keys, constructed from lengthat-age data presented by Griffiths (1996), were used to transform length frequency distributions to age frequency distributions (Butterworth et al., 1989). Instantaneous total mortality (2) was then estimated by catch curve analysis (Ricker, 1975); and secondly Total annual

mortality

was estimated

odorus in each of the three study regions

SOUTH

AFRICA

I 26

Fig. I. The three regions and the localities mentioned in the text.

280

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M.H.

106

Grijfiths/

Fisheries

Table 1 Parameters from Griffiths (1996)*.’ and Griffiths (1995)’ the per-recruit analyses for A. inodorus Parameter

L(mm) K (year-

SC

SEC 1141.8 0.65 6.93 0.26

‘1

t* (years) P 6X

1O-6 3.07

1 t,,,

6X

(years)

t,, (years)

25 1.3

Source

swc .1141.8 0.65 6.93 0.26 1O-6

3.07 1.5 2.4

used in

1086.8 0.41 3.34 6xlO-6

3.07 12 1.9

equation

(Butterworth

30 (1997)

103-115

where r,,, is the age-at-50% Efanov, 1977),

A A A A B B c

1.521

- 0.155

(3)

(Gunderson

(fish weight - ripe gonad weight)

and Dygert,

1988),

M = 2.88 x W-o.33 where W is fish weight converted (Boudreau and Dickie, 1989),

et al.,

ln( 2) = 1.46 + - 0.01 X ln( t,,,)

(2)

(4) to kilocalories

(5)

where rmax is the maximum age sampled (Hoenig, 1983). In the case of Eq. (4), M was calculated for each age (from weights corresponding to calculated lengths-at-age) and the average of all ages was then computed. Weight was converted to kilocalories using the relationship derived for Argyrosomus juponicus (as A. holotepidotus) by Marais (1990). Although Eq. (5) was developed to estimated total mortality (Z), the empirical data on which it was derived pertained to unexploited or lightly fished stocks (Hoenig, 1983). Furthermore, although the numbers of older individuals in sciaenid stocks are reduced by fishing, some fish appear to survive to near maximum age, even under high levels of exploitation (Murphy and Taylor, 1989; Ross et al., 1995; Griffiths and Hecht, 1995; Griffiths, 1996). Taking into account these facts and the fact that considerable effort was made to sample the widest possible range of A. inodorus ages (Griffiths, 19961, the value determined using Eq. (5) is probably closer to M than to Z. Having estimated Z and M, fishing mortality was obtained by substitution (F = Z - M). 2.3. Per-recruit

analysis

Y/R and SB/R were determined using the microcomputer package PC-YIELD (version 2.2) (Punt, 1992): Y/R=/mS,xFxN,xW,df 0

.o.72

(Rikhter and

M = 0.03 + 1.68 x GSI

where GSI =

where a, is the age at full recruitment and a, is the mean age of all fully recruited fish sampled. An average of the two values (for each region) was used in subsequent analyses. Accurately calculating the natural mortality (M) of exploited fish populations is notoriously difficult. Pauly’s empirical equation (Pauly, 1980), based largely on the Von Bertalanffy growth curve parameters (K and L,), is the method most commonly used to obtain an estimate of this parameter (Beverton, 1992; Butterworth et al., 1989). Unfortunately the Von Bertalanffy model fitted poorly the observed length-at-age data of all three A. inodorus stocks (Griffith& 1996). As a result the methods of Pauly (1980) and also of Roff (1984) were unsuitable for South African silver kob. The following equations were, therefore, used to obtain an indication of M for these stocks:

M=

maturity

ripe gonad weight

The growth parameters (A) for the Southeastern Cape (SEC) and the Southern Cape (SC) pertained to the Richards function (Schnute, 1981) and for the Southwestern Cape @WC) to the Logistic (Schnute, 1981) function. t,, age at first capture; t,,,, a maximum age of 25 years was assumed for all stocks in subsequent analyses; tn,,age at 50% maturity.

from the following 1989):

Resenrch

SB/R

= j’““‘N, x W, dr ‘Ill

(7) (8)

M.H. Grijiths/

Fisheries Research 30 (1997) 103-115

where N, is the number of fish at Age t divided by the number of recruits, W, is the mass of fish of Age t, S, is the selectivity of the fishing gear on fish

107

of Age t, F is the instantaneous rate of fishing mortality on fully recruited cohorts, and t,,, and t,,, are age-at-50%-maturity and maximum age respec-

9

Cape

South-Eastern

8 7 6 5 4 3

3 5 % lz r lJ

2 1 0 1

3

5

7

9

11

13

15

17

19

21

23

25

1

3

5

7

9

11

13

15

17

19

21

23

25

8

South-we.stern Cape

7 n = 8 974

6

4

,x 2 S ;

3

2

5

2 1 0 1

3

5

7

9

11

13

15

17

19

21

23

25

Age scars)

Fig. 2. Regional age distributions (bars) and catch curves (circles) for A~~y~sonru.~ inodot~~ landed by the South African 1990- 1994. The slope of the descending limb of the catch curve being the estimate of total mortality (2).

linetishery,

108

tively. Recruitment a knife-edged i.e.

M.H. Griffirhs/

Fisheries Reseurch 30 (1997) 103-115

to the fishery was assumed to be

Table 2 Estimates a of instantaneous African A. inodorus Total mortality (2) Eq. (1) Cc

where t, is the age-at-first-capture, and was taken as the age corresponding to the top of the catch curve (Butterworth et al., 1989). Because ages at 100% recruitment were masked by high mortality rates after age three (see Fig. 2), it was not possible to accurately model the functional form of silver kob recruitment to the linefishery. Given the difficulties in obtaining accurate estimates of natural mortality for exploited populations, the sensitivity of the per-recruit models to various estimates of M was also tested. F,,, was calculated by PC-YIELD and FsB40 was determined graphically. 2.4. Catch trends South African commercial linefishers have been compelled to submit daily returns of catch and effort since 1986. CPUE (kilogram per boat per day) was plotted on an annual basis in an attempt to verify the results of the per-recruit analysis and to provide information that would assist with the formulation of effective management strategies.

3. Results Age distributions in the three South African silver kob stocks and corresponding catch curves are illustrated in Fig. 2. The current age at first capture was estimated as 3 years for all three stocks. Estimates of Z and M obtained using the catch curve and Eq. cl), Eq. (2), Eq. (3) and Eq. (4) are given in Table 2. Average Z was highest in the Southeastern Cape, lowest in the Southern Cape and intermediate in the Southwestern Cape. The estimates of M obtained using the method of Rikhter and Efanov (Eq. (2)) were unrealistically high (M > Z), and were disregarded in subsequent analyses. Eq. (31, Eq. (4) and Eq. (5) provided estimates of M = 0.10, 0.16 and 0.16, respectively. Previous studies show that Eq. (4)

SEC 0.81 0.49 SC SWC 0.65

0.84 0.65 0.57

total and natural mortality

for South

Natural mortality (Ml

Average Z F.q. (2) Eq. (3) Eq. (4) Eq. (5) 0.82 0.57 0.61

1.10 0.65

0.10 0.10 0.10

0.16 0.16 0.16

0.16 0.16 0.16

a Using the catch curve (Cc) and Eq. (1). Eq. (2). Eq. (31, Eq. (4) and Eq. (5). SEC, Southeastern Cape; SC, Southern Cape; SWC, Southwestern Cape.

has produced realistic estimates of M for at least two other sciaenids (Ross et al., 1995). According to Clark (19911, estimates of M for groundfish species are generally around 0.2. The instantaneous natural mortality of A. inodorus was therefore assumed to be between 0.1 and 0.2. Yield-per-recruit and spawner biomass-per-recruit curves for the three stocks of South African A. inodorus at the current age at first capture (t, = 3 years) and at three alternative values of natural mortality within the predicted range (M = 0.1, 0.15 and 0.21, are shown in Fig. 3. Although maximum yieldper-recruit fluctuated widely with M, there was little change in the estimates of F,,, and FSBM. Depending on the value of M, FsBbO was 0.10-0.13 for all three regions; and F,., was 0.09-o. 12 for both the Southeastern Cape and the Southern Cape, and O.lO0.16 for the Southwestern Cape. Estimates of FsBZS were 0.16-0.23 in each of the three regions. Fishing mortalities were estimated to be 0.62-0.72 for the Southeastern Cape, 0.37-0.47 for the Southern Cape and 0.5 l-0.61 for the Southwestern Cape; indicating that all three stocks are exploited way beyond the threshold. The spawner biomass-per-recruit curves for each of the three regions declined rapidly with F. Spawner biomass-per-recruit at current levels of F was estimated to be 2.9-9.8%(SB/R),= 0 (M = 0.1-0.2) in the Southeastern Cape, 6.512.5%(SB/R),= 0 in the Southern Cape and 4.4Cape. The 10.4%(SB/R),= 0 in the Southwestern spawner biomass-per-recruit curves and associated biological reference points (i.e. FsBz5 and F,,,,) were identical for silver kob in the Southern Cape and the Southeastern Cape (Fig. 3) indicating that the

M.H. Griflths/Fisheries

Research

model is insensitive to small changes (in this case 4.4% of t,,,) in age at maturity (silver kob in these two regions had identical growth rates but different

30 (1997)

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109

16

0.4

0.2

0.6

0.6

1.0

1.2

16

6 4 2 M

50

a-

0.10

40

;:

go”

I^

FO.1 kw 0.09 0.10 0.V

Southwestern

0.10 0.11 0.13

km

Fc,,mm

0.16 0.19 0.23

02

0.47 0.42 0.37

Cave

FI%NG

MORTAL%

0.8

Fig. 4. Isopleths describing the response of yield-per-recruit to different combinations of fishing mortality and age at recruitment for Argyrosomus inodorus in (a) the Southeastern Cape and the Southern Cape, and (b) the Southwestern Cape. M = 0.15. Current F and the t,s providing maximum Y/R are illustrated with dotted lines. I

2500

2000 rr1500 s 1000 500 M

a- 0.10 b 0.15 c- 0.20

0.2

F0.t 0.10 0.13 0.16

FSEMO Fsszs 0 to 0.16 0.11 0.19 0.13 0.23

0.4 FISHING MORTALITY

bmen~ 0.51 0.46 0.41

0:6

Fig. 3. Yield-per-recruit (Y/R) and spawner biomass-per-recruit @B/R) against fishing mortality (with biological reference points) for Argyrosomus inodorus in (a) the Southeastern Cape, (b) the Southern Cape and (c) the Southwestern Cape, at different levels of natural mortality M.t, = 3 years. See Table 1 for other input parameters. Fcurrcn, = current fishing mortality.

ages at maturity, Table 1). This suggests that the results obtained for the Southwestern Cape would not be improved by a more accurate estimate of age at maturity for this region (unless of course it was substantially different to the average value used in this study, which is unlikely). Isopleths describing the response of yield-per-recruit to different values of F and t, are shown in Fig. 4 for A. inodorus in the Southeastern Cape, the Southern Cape and the Southwestern Cape. Since there were no differences between the growth rates and ages at recruitment in the Southeastern Cape and in thee Southern Cape, the yield-per-recruit responses for these two regions were identical. The Y/R responses were typical of other species of

M.H. Grifjriths/Fisheries Research 30 (1997) 103-115

110

similar age structure (Huntsman et al., 1983; Buxton, 1992). In all cases Y/R increased rapidly at low values of F for most of the range of I,. At low values of t, maximum Y/R was attained at correspondingly low values of F, above which Y/R declined. Maximum Y/R was attained at a recruitment age of 9 years and a F value of approximately 0.5 in the Southeastern Cape and in the Southern Cape (Fig. 4(a)), and at 7 years and F = 0.8 in the Southwestern Cape (Fig. 4(b)). The affects of altering t, on spawner biomass-per-recruit in each of the three regions are illustrated in Fig. 5. Due to the insensitivity of the model to small changes in t, (above), the SB/R curves for the Southeastern Cape and Southern Cape were identical. In all cases spawner biomass-per-recruit (or population growth potential; Goodyear, 1993) increased rapidly with t, at corresponding values of F. CPUE has fluctuated substantially over the past

Fig. 6. Annual CPUE (kilogram per boat per day) and fitted linear regressions for Argyrosomus inodorus landed by commercial linefishers in each of the three sampling regions (1986- 1995).

10 years (Fig. 6), but in all regions declining trend was evident.

a gradually

4. Discussion 4.1. Stock status

0.0

02

0.4 FISHING

0.6

0.B

MORTALITY

Fig. 5. Spawner biomass-per-recruit (SB/R) curves for different ages at recruitment (r,), for each of the three sampling regions.

For the purposes of the present study recruitment was assumed to be knife-edged. Due to variability in both hook size and in length-at-age, this assumption is often not realistic for linefisheries. However in the case of A. inodorus the juveniles do not occur on linefishing grounds (Griffiths, 1995), with the result that recruitment to the fishery occurs over a narrower age range than for other linefish species (e.g. Buxton, 1993). In addition Buxton (1992) found no appreciable differences between the per-recruit curves applied using knife-edged or logistical recruitment functions, to each of two South African linefish species with similar life-spans to the silver kob. The assumption of knife-edged recruitment is therefore not thought to have significantly biased the per-recruit curves calculated for A. inodorus. The results of this study show that the three stocks of South African A. inodorus are not only exploited at levels far beyond those corresponding to maximum long-term yield (i.e. FO,, and FsedO), but that threshold values of F (i.e. FSBz5) have also been substantially exceeded. In addition, the SB/R ratios of these stocks have been reduced to between 3% and 12% of the pristine condition. This is indicative of severe stock depletion and a greatly depressed

M.H. Grij‘iths/Fisheries

population growth potential. Smale (1985) demonstrated a steady downward trend in the national trawled catch of silver kob (as A. hololepidotus) (1968- 198 1), and also provided evidence suggesting that trawl catches in the Southeastern Cape crashed between 1967 and 1972. Since inshore trawlers catch mostly juveniles and young adults (Griffiths, 1995) and their catches are therefore an indication of recruitment success, this information supports predicted recruitment overfishing. Anecdotal evidence, based on interviews with linefishers, who generally catch adult silver kob (Griffiths, 1995), indicates that the silver kob spawner biomasses collapsed in the late 1960s to early 1970s in both the Southeastern Cape (Hecht and Tilney, 1989) and in the Southern Cape (P. Simms, Sea Fisheries Research Institute, personal communication), further corroborating the above conclusions. Biological data supporting excessive exploitation includes the high proportion of recruits (3 years) and the low numbers of silver kob older than 7 years (even though tmax = 25 years) in the South African line caught catches (Fig. 2). The depletion of the silver kob (and other linefish) resources can be correlated with a massive increase in fishing effort following the simultaneous introduction of motorised vessels, the construction of small boat harbours along the coast and the availability of echo-sounding technology, after the Second World War (Penney et al., 1989). Commercial CPUE data over the last decade indicate that the three South African silver kob stocks have continued to decline, albeit gradually, after this initial depletion. Life-history characteristics which may have contributed to the demise of A. inodorus stocks include longevity (Griffiths, 1996) and the fact that they form large aggregations at predictable localities (Griffiths, personal observation). It is important to note that the stocks declined despite a minimum size limit which provided protection from exploitation by linefishers until maturity. Catches made by inshore trawlers are comparatively low, and considering that only about 24% of their catch is less than the size at 50% maturity (Griffiths and Hecht, 1993), the capture of immature fish by this sector is unlikely to have caused the stock depletion. The higher F in the Southeastern Cape may be attributed to a less pronounced offshore dispersal in winter as a consequence of the narrower shelf area there (see Fig. 1);

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30 (1997)

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111

with the result that the silver kob in this region remain within the daily range of linefishers throughout the year (Griffiths, 1995). The current approach to the management of South African linefish consists of setting minimum size limits at the sizes at 50% maturity, and arbitrarily determining bag limits based on the perceived vulnerability of the species to exploitation (Penney et al., 1989). It has been clearly demonstrated that reef fishes including the Sparidae, Serranidae and Lutjanidae, are particularly sensitive to exploitation by linefishing (Crowley, 1983; Huntsman et al., 1983; Buxton, 1992). One of the objectives of the South African bag limit system was therefore to redirect fishing effort from vulnerable reef fishes to more ‘resilient’ shoaling species (Penney et al., 1989). Perceived as a resilient species, catches of A. inodorus (as A. hololepidotus) were not limited when bag limits were first introduced to the linefishery in 1984 (Government Gazette No. 9543 of 31 December 1984). This was amended by a generous recreational bag of 10 fish man- ’ day- ’ in 1992 (Government Gazette No. 14353 of 23 October 1992), but only to curtail the sale of fish by this sector. Commercial catches remain unlimited. The current minimum size limit of 400 mm total length was first introduced in 1940 (Notice No. 1696, Government Gazette Vol. 122 of 25 October 1940). Although determined arbitrarily, a recent study (Griffiths, 1995) showed that it does provide protection from the linefishery until maturity (Southeastern Cape female L,, = 310 mm, Southern Cape female L,, = 375 mm; Griffiths, 1995). The results of the present study therefore show that silver kob are not nearly as resilient to linefishing as previously believed, and that the South African stocks have collapsed despite a minimum size limit that has provided protection for immature fish for more than twice the life-span of the species i.e. 55 years. The current approach to linefish management in South Africa therefore does not provide sufficient protection for species such as the silver kob. 4.2. Management

options

Although there is no clear statement as to whether biological or socio-economic considerations should be applied in the management of the South African

112

M.H. Grijfithirhs/ Fisheries Research 30 (1997) 103-l 15

linefishery, it is assumed that initial objectives for A. inodurus management should be to rebuild stocks to levels above those corresponding to Fs,,,. At a recent conference on the topic of reference points and fisheries management, it was concluded that the biologically defined threshold “must be beyond compromise to any social, economic or political objective as it concerns the very viability of the resource” (Smith et al., 1993). It must also be stressed that threshold reference points should not be accepted as TRPs (due to large associated risk) and every effort should be made to maintain stocks above the relative SB/R ratios (or biomass levels) associated with the former (Mace and Sissenwine, 1993). Stock rebuilding typically requires initial effort/catch reductions in order to increase future long-term yields and biomass levels; with short term losses in economic performance offset by long-term gains (Overholtz et al., 1993). Minimum size and bag limits constitute the two most effective methods of effort regulation for the multi-species, multi-user South African linefishery, particularly where migratory species such as the silver kob are concerned 1995). Commercial and recreational (Griffiths, lineboat fishers often argue against size and bag limits on the grounds that, due to the multispecies nature of the fishery, fish are accidentally caught in excess of species specific quotas, and, because of the effects of barotrauma, these and undersized fish often die when returned to the water. However, A. inodorus generally forms shoals of similar-sized fish which are mostly targeted for on specific grounds. It is therefore possible for lineboat fishers to move away from such areas once bag limits are reached or if the captured fish do not satisfy the minimum legal size limit. In addition most of the South African A. inudorus catch is made in < 60 m depth, and, while depth related mortality rates for released fish have not been calculated, an ongoing mark-recapture programme suggests that a substantial proportion of silver kob caught at depths of up to 50 m will survive if released. Based on the results of this study, the management strategies designed to rebuild South African A. inodorus stocks may be divided into four options. The first and most simplistic of these would be to institute a moratorium on A. inodorus until stocks recovered to more acceptable levels. Although this

strategy has been affectively applied to North Sea herring (Leaman, 1993) and there is currently a moratorium on Canadian cod stocks (Walters and Maguire, 1996), the importance of silver kob to the South African linefishery (even if depleted) suggests that a bag limit of zero would not be acceptable from the sociological viewpoint. The second option would be to maintain the current t, (3 years) but to reduce F by introducing suitable bag limits. A F of 0.15 (i.e. > FSBdO but < FSBz5) would entail effort (and catch) reductions of 76-79% (M = 0.10-0.20) for the Southeastern Cape stock, 60-68% for the Southern Cape and 71-75% for the Southwestern Cape. One of the problems with the implementation of bag limits in the South African context is that they are sometimes difficult to police. For example, catches in excess of bag limits may be transferred at sea, and, if a boat is not inspected, its catch can be offloaded and a second trip made. Also, once catches have changed hands (fisher to dealer), there is no means of determining the catches of individual vessels. In addition, it is difficult to translate target levels of F into appropriate bag limits (Punt, 1993), a task further complicated by the increasing effort associated with a rapidly expanding recreational sector (Van der Elst, 1989). As a result, intensive monitoring, regular estimates of F and continual bag size adjustments will be necessary with this option. The third option is to attempt target SB/R levels through adjustments in rc alone (i.e. no bag limit = 2). Responses of yield-per-recruit to different values of F and t, indicate that maximum yield would be attained by increasing the age at first capture from 3 years to 9 years in both the Southeastern Cape and the Southern Cape, and from 3 years to 6/7 years in the Southwestern Cape; with corresponding Fs of close to or/and larger than current values (Fig. 4). The SB/R models indicate that the new r,s would maintain SB/R at around 4O%(SB/R),= 0 at current values of F, and that even at higher fishing levels SB/R would not drop below 25%(SB/R),,, (Fig. 5). This strategy would require increases in the minimum size limit from 400 mm TL to 958 mm TL in the Southeastern Cape and the Southern Cape, and to 882 mm TL (6.5 years) in the Southwestern Cape. Although this strategy does not require bag limits, and also should maximize long-term yield, it would

M.H. Grifjths/

Fisheries

exclude most of the current hook and line catch (Fig. 2), and consequently may not meet current sociological requirements. The fourth option consists of a combination of both bag and size limits; which is possibly the most appropriate for South African A. inodorus. An important advantage of this option is that it allows for user group participation (i.e. decision on bag/size limit combination), which should enhance management performance and increase compliance with control measures (O’Boyle, 1993). Due to the difficulties of translating target Fs into equivalent bag limits, and since bag limits could be illegally exceeded, it is suggested that FsBaObe accepted as the target in order to ensure that true F is maintained substantially lower than F,,,,. An example of this type of strategy would be to reduce F to 0.2 in all three regions and to increase t, to 5 years (721 mm TL) in the Southwestern Cape and to 6 years (675 mm TL) in the Southern Cape and in the Southeastem Cape (see Fig. 5). Based the length weight relationship (Griffiths, 1996) and on the size frequencies of A. inodorus in the linefisheries of each region (1990- 1994)(Griffiths, 1995) these increases in minimum size would result in catch reductions of about 49% in the Southwestern Cape, 55% in the Southern Cape and 83% in the Southeastern Cape. Additional reductions (SW Cape = 56%, S Cape = 52% and SE Cape = 70%) in order to achieve F = 0.2 would result in initial annual catches of approximately 23% (SW Cape), 22% (S Cape) and 5% (SE Cape) of what they were between 1990 and 1994. Although this represents a large initial reduction in catch, significant catch increases are expected once the first unexploited cohorts reach the new t, (i.e. 2-3 years, which is the difference between old and new t,s), and also as recruitment levels increased in symphony with the spawner biomass. Excluding the catch increases associated with higher recruitment levels, the yield from each cohort is expected to increase X 2.3 in the Southeastern Cape, X 1.8 in the Southern Cape and X 1.3 in the Southwestern Cape (Fig. 4). Although it is not possible, without knowledge of the spawner-recruit relationship, to predict accurately the catch improvements associated with the target levels of relative spawner biomass; empirical and stochastic theoretical studies, based on a wide range of spawner recruit relationships (Clark,

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1993, Mace and Sissenwine, 1993, Mace, 1994), indicate that increasing the relative spawner biomass-per-recruit ratios of South African silver kob to 4O%, would result in substantial increases in recruitment and therefore catch. Due to the difficulties of translating target effort levels into bag limits (Punt, 1993), the relationship between the two can only be established by trial and error. Current research suggests that fishing mortality can be relatively quickly and reliably estimated for A. inodorus using tagging data (unpublished). 4.3. Conclusions The results of this study show that A. inodorus is not the resilient species that it was once perceived to be; and that South African stocks have collapsed despite a minimum size limit which protected individuals until maturity. This indicates that manual linefishing methods (i.e. handline and rod and reel) can deplete stocks of migratory as well as resident marine teleosts, if they are not managed correctly; and that optimal minimum size limits are often substantially larger than the sizes at 50% maturity. It is therefore suggested that where possible every attempt should be made to base bag and size limits for South African linefishes on per-recruit analyses (particularly SB/R). FsBhO and FSBT5 are respectively recommended as appropriate target and threshold reference points.

Acknowledgements I would like to thank the technical staff of the Sea Fisheries Research Institute (Cape Town) for their assistance with the collection of regional length frequencies. Anesh Govender of the Oceanographic Research Institute (Durban) and two anonymous referees commented constructively on earlier drafts of the paper. This research was funded in part by the Sea Fisheries Fund.

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