Vital population statistics of the exploited eel stock on the Swedish west coast

Fisheries Research 40 (1999) 251±265

Vital population statistics of the exploited eel stock on the Swedish west coast H. SvedaÈng* È regrund, Sweden National Board of Fisheries, Institute of Coastal Research, Gamla SlipvaÈgen 19, S-740 71, O Received 9 April 1998; accepted 23 October 1998

Abstract The population dynamics and exploitation of the yellow eel (Anguilla anguilla (L.)) stock on the Swedish west coast were studied. In contrast to a generally observed reduction in the recruitment of glass eels in Europe, including in Swedish waters, there was no indication of a decline in the total eel ®shery yields along the Swedish west coast. Long-term records of daily catches as well as by test ®shing results also shown that this stability in eel ®shery yields has not been maintained by an increase in ®shery effort, as the catch-per-unit-efforts in the past 20 years have been more or less unchanged. These ®ndings implied that the number of recruits to the ®shery has been rather stable, possibly indicating that density-dependent factors at the elver and yellow eel stages may moderate variations in glass eel recruitment. Total instantaneous rate of mortality was estimated from records on eel length distribution in the professional fyke-net ®shery. The estimated total mortality rate in an isolated archipelago population on the west coast was chosen as an approximation of the instantaneous rate of natural mortality and net emigration in the west coast eel stock. The differences between these two estimates could, thus be regarded as the mortality that occurred due to ®shing. It was found that the eel ®shery was very intense and most ®sh were caught in small sizes, resulting in a low escapement rate of maturing ®sh. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Anguilla anguilla; Catch-per-unit-effort; Escapement; Fishing mortality; Natural mortality

1. Introduction The eel ®shery (Anguilla anguilla (L.)) is still thriving on the Swedish west coast inspite of the reduced glass eel recruitment to Europe (e.g. Moriarty, 1990; Moriarty and Dekker, 1997). Since the eel stock in these coastal waters has not been strengthened by supportive stockings, it is obvious that the level of natural recruitment is still high enough to give rise to an eel stock worth exploiting. *Tel.: +46-173-46469; fax: +46-173-30949; e-mail: [email protected]

This eel ®shery is entirely based on a locally occurring stock of yellow eels, since silver eels migrate from the coast westwards, leaving little opportunity for ®shing along the migration routes as is done on the Swedish east coast (c.f. MaÈaÈr, 1947). Small fyke-nets, usually joined together into long links, are the preferred ®shing gear. Trawling or other active ®shing methods are not employed. Eel ®shing is restricted to the sheltered parts of the coast, that is from the northern part of the county of Halland to the Norwegian border (Fig. 1), as the open coastal area in the southern part of Halland is considered sub-optimal for eel ®shing. The minimum size limit on the Swedish

0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00226-4

252

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

Fig. 1. Map showing areas where daily records of fishing yields and records of the length distributions were made, and where the monitoring test fishing was performed on the Swedish west coast.

west coast is set at 370 mm in total length; eels between 370 mm and about 490 mm in length are thus marketed for stocking purposes (Holmgren and WickstroÈm, 1988), for example in inland waters and in the Baltic Sea. Concerns have been expressed as to the viability of the European eel population (SjoÈstrand, 1996; Moriarty and Dekker, 1997), as there is considerable evidence of major recruitment failures in recent decades (Erichsen, 1976; SvaÈrdson, 1976; Moriarty, 1990;

Lara, 1994; Moriarty and Dekker, 1997) which has also been re¯ected in recruitment indices from the Swedish west coast (HagstroÈm and WickstroÈm, 1990; SvedaÈng, 1996a). Over®shing and a low escapement rate, eventually leading to a reduced spawning stock biomass, have been recognized as a possible threat to the important eel ®shery and aquaculture in Europe (estimated at a total value of 180 M ECU, plus 360 M ECU in added value, Moriarty and Dekker, 1997), and possibly also to the very survival of the species. In order to ensure appropriate management of eel resources, population dynamics and the exploitation rate of the eel stocks on the Swedish west coast were studied. Eel catches in the professional ®shery were examined, including records on catch-per-unit-effort (CPUE), size and age distributions, growth and total yield. In addition, in order to evaluate trends in CPUE, results from monitored test ®shing in the northern part of Halland were included. The instantaneous rate of natural mortality and net emigration was estimated by studying an almost unexploited eel stock in the Koster Islands, as these islands form an isolated archipelago outside the Swedish mainland off the northern part of the west coast. The only eel ®shing known to take place around these islands is a single, small-scale ®shery. It was also recognized that the eel stock was probably more or less isolated from the mainland, due to the considerable depth of water surrounding this archipelago. The absence of the Asiatic swim-bladder parasite Anguillicola crassus at Koster, in contrast to its presence among eels stocks along the Norwegian and Swedish mainland (SvedaÈng, 1996b), adds support to the theory of a virtually cut-off stock of yellow eel in the Koster Islands. 2. Material and methods 2.1. Collection of data 2.1.1. Catch statistics Of®cial statistics from the eel ®shery in the Swedish part of Skagerrak and Kattegat were available from 1914 to 1995 (SCB, 1995). Besides the of®cial statistics, daily catch records of professional ®shermen using small fyke-nets, were analysed. Fyke-nets used in eel ®shing on the west coast have a mesh size (knot

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

to knot) of 10±11 mm at the cod end. In the county of Halland, three areas were monitored between 1976 and 1994, that is Bua, VendelsoÈfjorden and Kungsbackafjorden (Thoresson, 1976; 1992; Fig. 1). Most of the time, one or two ®shermen were engaged concurrently within each area. The number of areas where daily catches were recorded was extended in 1994 to include more northerly locations in the county of BohuslaÈn, namely, FjaÈllbacka and Marstrand, with two or three ®shermen recording their catches within each area (Fig. 1). This material might be considered as more reliable than the of®cial catch statistics as the information given by the ®shermen has been treated as strictly con®dential. This information was analysed for CPUE (weight of eels per unit effort). Time trends within, as well as between, areas were examined by calculating the unweighted mean value of the annual mean CPUE for all the ®shermen within the same area (Table 1). In calculating CPUE, the unit effort equalled the number of fyke-nets, as the exact number of days between setting and examining was unregistered. Hence, to check whether changed ®shing methods might have reduced variations in CPUE due to, for

253

instance, variations in recruitment, time trends were also examined in the total annual yield, the total annual number of fyke-nets efforts and the number of days per season when examinations occurred. 2.1.2. Test fishing Standardized annual test ®shing with fyke-nets was carried out at six stations in VendelsoÈfjorden south of Gothenburg between 1976 and 1997 (Thoresson, 1992). Test ®shing was repeated on 12 occasions per station in August using two fyke-nets. The mean yearly CPUE was calculated per station according to the mean number of eels caught per fyke-net and ®shing occasion (Table 2). 2.1.3. Length and age distributions in professional eel fishery catches To estimate the size distribution in the eel fyke-net ®shery, collaborative work was initiated with some professional ®shermen in 1993 at four different locations on the west coast (VendelsoÈfjorden/ Horta, Marstrand, FjaÈllbacka and the Koster Islands, Fig. 1). In each locality, one or more catches for examination

Table 1 Unweighted mean values of mean annual CPUE of individual fishermen within area, based on daily records of the catch of eels by weight (kg) per fyke-net, at different sites on the Swedish west coast Year

Horta (CPUE)

VendelsoÈfjorden (CPUE)

Kungsbackafjorden (CPUE)

1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996

0.24 0.19 0.18 0.18 0.20 0.17 0.18 0.23 0.28 0.17 0.14 0.16 0.30 0.26 0.29 0.20 0.27

0.27 0.31 0.49 0.30 0.26 0.32 0.25 0.26 0.27 0.21 0.21 0.19 0.24 0.29 0.42 0.24 0.31 0.51 0.33

0.45 0.58 0.48 0.38 0.29 0.44 0.64 0.61 0.69 0.64 0.29 0.23 0.36 0.28 0.32 0.36 0.33 0.39 0.30

Marstrand (CPUE)

FjaÈllbacka (CPUE)

0.33 0.32 0.24

0.62 0.58 0.54

254

Table 2 Mean number of eels caught per fyke-net and night in August in a test fishing in VendelsoÈfjorden 1976±1978 and 1981±1997 at six fixed stations (Thoresson, 1992) Year

Station 42 (CPUE)

Station 43 (CPUE)

Station 44 (CPUE)

Station 45 (CPUE)

Station 46 (CPUE)

Unweighted mean value for all stations

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

0.55 0.07 0.05 0.58 0.25 0.09 0.37 0.20 0.32 0.32 0.61 0.61 0.19 0.50 0.29 0.33 1.88 0.75 0.13 0.14

0.38 0.11 0.15 0.12 0.25 0.20 0.44 0.35 0.37 0.46 0.76 0.45 0.56 0.59 0.38 0.58 1.72 0.87 0.31 0.23

0.14 0.13 0.02

0.2 0.15 0.08

0.26 0.07 0.09

0.35 0.11 0.17

0.10 0.17

0.02 0.04

0.07 0.09

0.22 0.49 0.71 1.09 0.66 1.05 0.60 0.60 0.19 0.80 0.54 1.05 1.01 0.66 0.26 0.47

0.13 0.09 0.27 0.55 0.75 0.32 0.25 0.57 0.42 0.54 0.21 0.29 0.67 0.88 0.25 0.14

0.22 0.30 0.37 0.60 0.58 0.42 0.38 0.62 0.34 0.41 0.38 0.45 0.78 0.96 0.58 0.23

0.33 0.11 0.34 0.12 0.11 0.34 0.88 0.44 0.70 0.73 0.86 1.05 0.60 0.58 0.58 0.81 3.71 0.69 0.19 0.32

0.04 0.09

0.13 0.41 0.40 0.70 0.64 0.86 0.64 0.54 0.08 0.82 0.54 0.63 1.46 0.75 0.21 0.45

0.31 0.04 0.23 0.08 0.05 0.18 0.67 0.55 0.68 0.79 0.86 1.21 0.27 1.21 0.82 0.54 2.17 0.67 0.08 0.14

0.47 0.30 0.40 0.25 0.54 0.32 0.61 0.21 0.19 0.54 0.21 0.42 2.29 1.00 0.17 0.59

0.07 0.40 0.51 0.42 0.46 0.46 0.90 0.26 0.43 0.46 0.26 0.51 2.02 1.30 0.33 0.80

0.23 0.18 0.27 0.40 0.54 0.39 0.89 0.71 0.38 1.04 0.14 0.67 1.67 0.25 0.54

0.43 0.46 0.37 0.52 0.50 0.74 0.86 0.64 0.55 0.89 0.23 1.05 1.97 0.34 0.86

0.32 0.06 0.09 0.33 0.21 0.21 0.40 0.44 0.58 0.50 0.64 0.64 0.25 0.78 0.37 0.48 1.69 0.77 0.23 0.29

0.17 0.04 0.08 0.35 0.15 0.13 0.15 0.19 0.15 0.25 0.23 0.33 0.13 0.30 0.26 0.16 0.59 0.35 0.16 0.22

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

1976 1977 1978 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

Station 41 (CPUE)

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

were kept alive `in corf' (i.e. submerged netbags) until measurements could be made by staff from the Institute of Coastal Research. The total length (in cm) of every ®sh caught was measured. Because the catch had not been sorted according to the minimum size limit, it, therefore, included ®sh smaller than 370 mm. Eel catches were examined in autumn 1993, and in spring and autumn 1994 and 1995, except in the Koster Islands where the length distribution was recorded only in autumn. In the following two years, 1996 and 1997, details were recorded on several occasions during the ®shing season, that is from May to October/November. The programme was extended in 1997 to include southern Norway as well as two ®shermen on the islands of Kirkùy and érnekupa (Fig. 1). To study the age, weight distribution and sex ratio, about 100 eels were occasionally sampled at random from the catch, and killed by deep-freezing before dissection some weeks later. The samples were then measured (weight in grams and length in mm), sexed and a random selection was also age estimated. In samples gathered in 1993 and 1994 the age was determined from ocular otolith readings (c.f. Holmgren and WickstroÈm, 1988). To obtain a sagittal view, the otolith was embedded convex side up in thermoplastic (thermoplastic quartz cement; No. 70C Lakeside Brand) on a microscope slide. This side and sometimes also the other side was ground with a series of wet-grinding papers (600, 800 and 1200 grade) with a constant supply of water. The ground preparation was then read under a light microscope. Evaluation of the grinding method has shown that this method is as useful and valid as other methods such as burning and cracking (Holmgren and WickstroÈm, 1988; Vùllestad et al., 1988).

255

the otolith increase was measured at this angle by means of a digitizer and a light microscope. The length-at-age for each individual ®sh was back-calculated according to the equation (Thoresson, 1996): r  L ˆ Lt R where L is the total length of the ®sh, Lt is the backcalculated length at age t, R is the caudal radius length (0.01 mm), and r is the intermediate otolith radius. The back-calculated length-at-age showed clearly that the growth rate varied between eels, but also that the individual length growth pattern was linear (Fig. 2; mixed nested anova, model adjusted R2 ˆ 0.86; Table 3). Such linear growth rate patterns have also

3. Analysis of data 3.1. Growth rate To estimate the mean growth rate of the eel stock on the Swedish west coast, age-at-length was back-calculated in some ®sh, that is from FjaÈllbacka (n ˆ 49) and from the Koster islands (n ˆ 47). As a good linear correlation with ®sh length has been established for the caudal otolith radius (e.g. Rossi and Villani, 1980),

Fig. 2. Back-calculated length-at-age for eels caught in the (a) Koster islands (n ˆ 47) and in FjaÈllbacka (n ˆ 49) in 1993.

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

256

Table 3 Mixed nested ANOVA results, including area, individual fish (Type III sum of squares), for the deviations in length-at-age with age as covariate Source

Sum of squares

Intercept

Hypothesis Error Hypothesis Error Hypothesis Error Hypothesis Error

Area Individual fish within area Agea

df 1013362.17 416454.026 44806.662 1061625.09 1165909.12 363068.7 8399669.72 363068.7

F-value

1 125.252 1 95.036 94 620 1 620

p

304.778

>0.001

4.011

0.048

21.181

>0.001

14343.829

>0.001

Adjusted model R2 ˆ 0.86 Coefficients of model 1 : B ˆ 46 mm/year, intercept: 76 mm.

a

been noticed in other ®eld studies (e.g. Vùllestad and Jonsson, 1986; Bergersen and Klemetsen, 1988) as well as in an experimental study on individually marked eel (Holmgren and Mosegaard, 1996). Hence, individual growth rates were established from lengthat-age relationships, including aged ®sh from all localities. The mean total length at the glass eel stage was set at 70 mm. The annual increment in length was calculated according to the following equation: G ˆ …L ÿ 70†=toto where G is the mean growth rate (mm per year), L is the total length of the ®sh and toto is the established otolith age. The estimate of the mean growth rate was based on all the age estimated female eels from VendelsoÈfjorden/ Horta, Marstrand, FjaÈllbacka and Koster in 1993 and 1994. The relationship between weight and length was estimated by loglinear regression analysis according to the equation: W ˆ a  Lb where W is the weight in grams. This relationship was estimated on all the eels sampled and analysed between 1993 and 1995 at all the investigated localities, that is VendelsoÈfjorden/ Horta, Marstrand, FjaÈllbacka and the Koster Islands. 3.2. The total and natural instantaneous rate of mortality The total instantaneous rate of mortality, Z, was estimated by catch-curve analysis (Ricker, 1975), by

means of linear regression of ln(relative number of ®sh per length class (i.e. 25 mm)) according to ®sh age within the length interval 400±625 mm. The length distributions were transformed into age distributions by estimating the mean age for each length interval; that is the increase in age between two length intervals equalled the increase in size divided by the estimated mean annual growth rate for female eels (G). It was assumed that the mortality rate was similar for all length classes, that the variation in recruitment to the ®shery was insigni®cant between the years and that the commercial stock was composed entirely of females. It was also assumed that there was no length related selectivity of the ®shing gear in the actual length interval. The estimated Z-value was given as Z ˆM‡F‡U where M is the instantaneous rate of natural mortality, F is the instantaneous rate of ®shing mortality and U is the instantaneous rate of other losses, that is above all, the silver eel migration to the Sargasso Sea. The estimated mean Z-value from the Koster Islands was considered as an appropriate approximation of the instantaneous rate of natural mortality and other losses at the Swedish west coast, that is M ‡ U. 4. Results 4.1. Catch statistics The annual ®shing yields in the Kattegat±Skagerrak area, that is, the counties of Halland and BohuslaÈn,

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

257

Fig. 3. Official records on the eel catch between 1914 and 1995 in the Swedish part of Kattegat and Skagerrak.

have ¯uctuated around 300±350 tonnes in the past three decades, with some notable exceptions at the beginning of the 1980s and in 1994, when catches exceeded 500 tonnes (SCB, Fig. 3). Daily catch records provided by professional ®shermen in three areas in Halland were analysed (Fig. 1). The unweighted annual mean value of CPUE of individual ®shermen was more or less unchanged between 1976 and 1994 in all the three areas (linear regressions). It could be concluded that changes in CPUE for different ®shermen occurred concurrently neither within nor between areas, and were both positive and negative (Fig. 4). It could be objected that the stable catch level might have been maintained by changing the ®shing habits. However, as the total annual yield as well as the total annual number of fyke net efforts and the number of days when examinations occurred were unchanged between 1976 and 1994 for every ®sherman (linear regressions; N.S.), it is likely that the overall ®shing pattern was of a similar kind from year to year. Nevertheless, it cannot be excluded that the total number of ®shing gear placed on the ®shing grounds at the same time might have changed, that is, the period of exposure of the single ®shing gear. This possibility is, however, less likely as it would imply that similar measurements have been taken by all ®shermen, which in the case of a longer period of exposure would lead to a sharp competitive situation on the limited ®shing grounds (pers. obs.); a situation which in itself probably would have led to reductions in CPUE.

The test ®shing results from the northern part of Halland also showed a more or less unchanged CPUE between 1976 and 1997, as a positive tendency could only be detected at two stations (Table 2, Stations 42 and 46; linear regressions; p < 0.05). It could also be observed that, although the time series was rather short, the CPUE-values were higher in the northern part of the eel ®shing area, that is at FjaÈllbacka, than at the other sites (Table 1). 4.2. The length and age distributions in the professional fyke-net fishery In all localities the catch analysis showed that the number of eels increased markedly with size upto about 400 mm in total length (Fig. 5). Above this size, ®sh numbers decreased consistently. This pattern was probably due to the fact that eels were fully recruited to this speci®c ®shing gear at a total length of about 400 mm. It should also be noted that about 25% of the total catch was smaller than the present minimum size limit of 370 mm (Table 4). It was also observed that the mean lengths of the eels caught in the fyke-net ®shery were smaller at VendelsoÈfjorden/ Horta, Marstrand and FjaÈllbacka, that is where eel ®shing is considered to be more intensive than in the Koster Islands. The youngest eels so far analysed, were 4±5 years old in all the localities (Fig. 6). This means that eels are recruited to the professional eel ®shery in their ®fth or sixth growth season. The number of eels in the

258

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

Fig. 4. The annual mean CPUE (in weight per fyke-net and examination) for individual fishermen between 1976 and 1994 in the northern part of the county of Halland.

catches was drastically reduced at higher ages. This could be because at this age eels start to mature and consequently `disappear' as they start migrating to the Atlantic spawning grounds. A recent study of silver eels from one of the areas included in this study (VendelsoÈfjorden) showed that eels in this brackish/ marine environment (20±30% PSU) were able to attain the silver eel stage at age 8‡, but that most were maturing between age 12‡ and 14‡ (SvedaÈng et al., 1996). The oldest eels observed in this study were caught in the Koster Islands (15‡) and VendelsoÈfjorden/ Horta (18‡), indicating the upper age limit for yellow eels on the Swedish west coast. 4.3. Growth and sex ratio The observed growth pattern was linear but the rate of growth varied between individual ®sh (Fig. 2 and Table 3). The mean individual growth rate (G) was estimated for females in the age class interval of 4‡ to 11‡ at 45 mm per year (n ˆ 494, SD ˆ 8.6 mm), according to the observed age-at-length relationships.

The relationship between length and weight was estimated as W ˆ 3.64  10ÿ7  L3.21 (n ˆ 1924; r2 ˆ 0.93; p < 0.0001). The proportion of males less than 370 mm in total length varied from 0 to 39% between sampling occasion and locality, and from 0 to 9% in the length interval between 370 and 470 mm, respectively (Table 5). 4.4. The total and natural instantaneous rate of mortality The total instantaneous rate of mortality (Z) was estimated on different sampling occasions by calculating the mean age of every length class with an assumed mean growth rate of 45 mm per year. The estimated Z-values for the eel stock around the Koster Islands varied to some extent between years and sampling occasions (Table 6). However, most estimates indicated similar, fairly low, mortality rates. The estimated mean value in the Koster Islands between 1993 and 1997, including all sampling occasions, was chosen as an approximation of a common instantaneous rate of natural mortality and

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

259

Fig. 5. The size distribution in the fyke-net eel fishery at different sites on the Swedish west coast and in Norway. The figures were taken from records of unsorted catches in the professional fishery between 1993 and 1997 at (a) VendelsoÈfjorden/Horta (n ˆ 2049), (b) Marstrand (n ˆ 4822), (c) FjaÈllbacka (n ˆ 1833), (d) Koster Islands (n ˆ 1268), and in Norway in 1997 (e) Kirkùy (n ˆ 985), (f) érnekupa (n ˆ 765).

260

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

Table 4 Mean length (mm) for eels longer than the minimum size limit of 370 mm total length based on records of unsorted fyke-net catches on the Swedish west coast between 1993 and 1997 Locality

The Koster islands

Unweighted mean value for 1993±1997 FjaÈllbacka

Unweighted mean value for 1993±1996 Marstrand

Unweighted mean value for 1993±1997 VendelsoÈfjorden/ Horta

Unweighted mean value for 1993±1997 Grand unweighted mean value for all localities except the Koster Islands 1993±1997

Year

1993 1994 1995 1997 1993 1994 1995 1996 1993 1994 1995 1996 1997 1993 1994 1995 1996 1997

(n)

136 179 216 651 4 140 297 266 378 4 231 304 341 1343 1293 5 246 237 419 633 214 5 3

Total length Mean

SD

505 478 511 493 497 489 465 461 467 470 493 461 464 448 455 464 467 417 467 452 446 450 460

84 77 85 82 15 86 67 78 78 13 92 71 70 74 67 17 75 36 67 64 54 20 10

Proportion of eels <370 mm 16 8.7 2.3 5.5 8.1 28 25 33 55 35 21 24 26 38 14 25 16 6.0 4.1 20 23 14 14

The proportion of undersized eels is given as a percentage (%).

other losses at the Swedish west coast, that is (M ‡ U) ˆ 0.23. The estimates of Z at the other sites, where eel ®shing was much more intensive, were higher than in the Koster Islands. It was also observed that estimates obtained in FjaÈllbacka varied somewhat more between years and sampling occasion than at the other sites on the Swedish west coast (Table 7). It was also evident that the mean Z between 1993 and 1997 increased from north to south along the coast, indicating a gradient in the instantaneous rate of ®shing mortality (F). It was noticed too, that differences in ®shing intensity coincided with similar differences in mean annual CPUE (Table 1). The estimated mean Z in Norwegian coastal waters, adjacent to Sweden, were higher than at FjaÈllbacka, but lower than recorded at the other Swedish localities. However, because three of the studied localities, VendelsoÈfjorden/ Horta, Marstrand and FjaÈllbacka, can be considered as represen-

tative of one third of the coastal eel ®shing areas along the Swedish west coast, an unweighted mean Z was calculated on the basis of the three mean values of Z between 1993 and 1997 at these localities; that is Zmean ˆ 0.54. Accordingly, a mean F was estimated by subtracting the estimate of natural mortality and emigration in the Koster Islands from Zmean, that is Fmean ˆ 0.31. The instantaneous rate of ®shing mortality, estimated locally, varied from 0.16 at FjaÈllbacka in the northern part of the west coast to about 0.42 south of Gothenburg. 4.5. Stock size The total number of eels on the Swedish west coast over 370 mm was calculated according to the equation (Ricker, 1975): C ˆ …1 ÿ eÿFmean †  Ntot

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

261

Fig. 6. The otolith age distribution of eels sampled from the fyke-net eel fishery at different sites in 1993 and 1994. a) VendelsoÈfjorden/ Horta, (b) Marstrand, (c) FjaÈllbacka, (d) Koster Islands.

where C is the total number of eels caught in the professional ®shery and Ntot is the total number of eels larger than 370 mm in the eel ®shing area along the Swedish west coast. The unweighted mean length of eels above the minimum size limit in the fyke-net ®shery was calculated at 460 mm at the three studied localities of known intensive ®shing (Table 4). Hence, the mean weight could be determined according to the known length and weight relationship to 128 g. Because the mean annual total eel ®shery yield between 1980 and 1995 can be calculated at 380 (SD 150 tonnes; SCB), the number of eels larger than the minimum size limit caught on the west coast can be estimated at about 3.0 million. According to the estimate of Fmean and the mean annual number of eels larger than 370 mm caught in the professional ®shery, the total number of eels above this size limit, Ntot, can, thus be estimated at about 11 million.

4.6. Recruitment to the fishery The number of ®sh that are recruited to ®shery can be estimated according to the equation (Ricker, 1975): tˆt Z

C ˆ Fmean

ReÿZ…tÿtR † dt

tˆtR

where R is the mean annual number of recruits larger than 370 mm to the ®shery, tR is the age of the ®sh at recruitment to the ®shery, t is the age of the ®sh when it leaves the ®shery on its spawning migration. Because the mean size of eels in this coastal region at the silver eel stage has been observed to be about 650 (SD ˆ 59 mm) in total length (SvedaÈng et al., 1996), the mean duration of the growth period in lengths between 370 and 650 mm can be calculated

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

262

Table 5 The proportion of males (%) in different size classes (less than 370 mm and between 370 and 470 mm in total length) on different sampling occasions (number of fish within parenthesis) and localities Locality

Year

Month

<370 mm

Koster

1993 1994 1995 1993 1994 1994 1995 1995 1993 1994 1994 1995 1995 1993 1994 1995 1995 1996 1997

September September August September May September June August September May August May August August August May September September August

0 0 0 3.8 0 9.1 0 3.8 7.4 7.7 39 22 19 11 0 0 0 4.5 14

FjaÈllbacka

Marstrand

VendelsoÈ-fjorden/ Horta

370  x < 470 mm (6) (9) (4) (26) (22) (22) (21) (53) (27) (13) (28) (9) (37) (9) (2) (3) (3) (22) (44)

0 2.2 0 2.6 0 3.6 0 0 7.5 2.4 9.4 7.0 6.5 7.5 8.8 0 0 4.7 6

(28) (46) (42) (39) (34) (56) (45) (37) (53) (41) (53) (43) (46) (53) (91) (39) (80) (64) (84)

Table 6 Total instantaneous rate of mortality, Z, according to the recorded length distribution in a fishery in the Koster Islands, estimated to be within the length interval of 400±625 mm by means of linear regression of ln(relative number of fish per length class) on fish age (calculated assuming a mean annual growth rate of 45 mm) Locality

Year

Month

R2

Degrees of freedom

F-value

p

Z

95% confidence interval

Koster

1993 1994 1995 1997 1997 1997 1997 1997 1993±1997

August August August May June August September All

0.53 0.81 0.29 0.19 0.71 0.80 0.21 0.40 0.42

7 7 7 7 7 7 7 34 61

7.78 29.2 2.91 1.66 17.2 28.9 1.84 22.5 43.8

0.0269 0.0010 0.1316 0.2392 0.0043 0.0010 0.2170 <0.0001 <0.0001

ÿ0.16 ÿ0.31 ÿ0.11 ÿ0.18 ÿ0.20 ÿ0.53 ÿ0.12 ÿ0.26 ÿ0.23

ÿ0.05 < x < ÿ0.27 ÿ0.20 < x < ÿ0.42 0.02 < x < ÿ0.23 0.10 < x < ÿ 0.46 ÿ0.11 < x < ÿ0.30 ÿ0.34 < x < ÿ0.73 0.05 < x < ÿ0.30 ÿ0.15 < x < ÿ0.37 ÿ0.16 < x < ÿ0.30

at 6.2 years, assuming a mean individual growth rate of 45 mm per year. The mean number of recruits to the ®shery can thus be calculated at about 5.4 million eels per year. 4.7. Escapement The proportion of the yellow eel stock that will survive to the silver eel stage (the mean silver eel size has been estimated at 650 mm; SvedaÈng et al., 1996), after having attained the minimum size limit of

370 mm, was estimated according to the equation (Ricker, 1975): S ˆ eÿZti where S is the survival rate and ti is the number of years to the silver eel stage, calculated at 6.2. Thus, after having been recruited to the ®shery, the estimated proportion of the eel stock that will survive to the mean silver eel size, can be estimated at about 3.5% or in other words, about 0.2 million eels. This number can be regarded as an estimate of the rate of silver eel

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

263

Table 7 Total instantaneous rate of mortality, Z, according to the recorded catch length distribution in the professional fishery on various sampling occasions at different localities on the Swedish west coast, estimated to be within the length interval of 400±625 mm by means of linear regression of ln(relative number of fish per length class) on fish age (calculated assuming a mean annual growth rate of 45 mm) Locality

Year

Month

R2

FjaÈllbacka

1993 1994 1994 1994 1995 1995 1995 1996 1996 1996 1996 1993ÿ1996

August May August All June August All June July August All

0.52 0.76 0.79 0.72 0.74 0.66 0.69 0.63 0.87 0.40 0.59 0.59

7 7 7 16 7 7 16 6 4 5 19 64

Marstrand

1993 1994 1994 1994 1995 1995 1995 1996 1996 1996 1996 1996 1996 1997 1997 1997 1997 1997 1993ÿ1997

August May August All June August All June July August September October All May June August September All

0.76 0.69 0.79 0.72 0.85 0.90 0.82 0.82 0.43 0.87 0.90 0.79 0.75 0.91 0.92 0.89 0.93 0.83 0.75

7 7 7 16 7 7 16 7 7 7 7 7 43 7 7 7 7 34 124

All

0.98

7

August August May September All September September October November All May August All

0.95 0.90 0.66 0.93 0.63 0.98 0.91 0.63 0.79 0.77 0.96 0.68 0.80 0.70

7 7 7 7 15 7 7 6 7 33 6 7 15 84

VendelsoÈÿ 1988 fjorden/Horta 1993 1994 1995 1995 1995 1996 1996 1996 1996 1996 1997 1997 1997 1993ÿ1997

Degrees of freedom

F-value

p

Z

95% confidence interval

7.58 22.2 26.5 40.4 20.1 13.8 35.9 10.4 27.8 3.38 27.7 90.7

0.0284 0.0022 0.0013 < 0.000 1 0.0028 0.0075 < 0.0001 0.0181 0.0062 0.1253 <0.0001 <0.0001

ÿ0.30 ÿ0.32 ÿ0.56 ÿ0.44 ÿ0.48 ÿ0.51 ÿ0.50 ÿ0.40 ÿ0.38 ÿ0.20 ÿ0.31 ÿ0.39

ÿ0.09 < x < ÿ0.52 ÿ0.19 < x < ÿ0.46 ÿ0.35 < x < ÿ0.78 ÿ0.31 < x < ÿ0.58 ÿ0.27 < x < ÿ0.69 ÿ0.24 < x < ÿ0.79 ÿ0.33 < x < ÿ0.66 ÿ0.16 < x < ÿ0.64 ÿ0.24 < x < ÿ0.51 0.01 < x < ÿ0.40 ÿ0.19 < x < ÿ0.43 ÿ0.31 < x < ÿ0.47

23 16 26 40 38 61 74 32 5.4 48 64 26 131 71 86 58 94 169 371

0.0020 0.0055 0.0014 <0.0001 0.0004 <0.0001 <0.0001 0.0008 0.0534 0.0002 <0.0001 0.0014 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001

ÿ0.31 ÿ0.46 ÿ0.59 ÿ0.52 ÿ0.40 ÿ0.64 ÿ0.52 ÿ0.73 ÿ0.35 ÿ0.70 ÿ0.82 ÿ0.70 ÿ0.66 ÿ0.37 ÿ0.78 ÿ0.77 ÿ0.64 ÿ0.64 ÿ0.59

ÿ0.18 < x < ÿ0.44 ÿ0.23 < x < ÿ0.69 ÿ0.36 < x < ÿ0.82 ÿ0.40 < x < ÿ0.64 ÿ0.27 < x < ÿ0.53 ÿ0.48 < x < ÿ0.80 ÿ0.40 < x < ÿ0.64 ÿ0.48 < x < ÿ0.99 ÿ0.05 < x < ÿ0.65 ÿ0.51 < x < ÿ0.90 ÿ0.62 < x < ÿ1.02 ÿ0.43 < x < ÿ0.96 ÿ0.55 < x < ÿ0.77 ÿ0.28 < x < ÿ0.46 ÿ0.61 < x < ÿ0.94 ÿ0.57 < x < ÿ0.97 ÿ0.51 < x < ÿ0.77 ÿ0.54 < x < ÿ0.74 ÿ0.53 < x < ÿ0.65

288

<0.0001

ÿ0.42

ÿ0.37 < x < ÿ0.47

126 53 14 79 25 274 68 10 27 112 138 15 60.6 199

<0.0001 0.0003 0.0075 <0.0001 0.0002 <0.0001 <0.0001 0.0187 0.0013 <0.0001 <0.0001 0.0060 <0.0001 <0.0001

ÿ0.46 ÿ1.14 ÿ0.28 ÿ0.91 ÿ0.57 ÿ0.78 ÿ0.49 ÿ0.72 ÿ0.56 ÿ0.63 ÿ0.80 ÿ0.68 ÿ0.74 ÿ0.65

ÿ0.38 < x < ÿ0.54 ÿ0.83 < x < ÿ1.45 ÿ0.13 < x < ÿ0.42 ÿ0.71 < x < ÿ1.11 ÿ0.35 < x < ÿ0.79 ÿ0.69 < x < ÿ0.88 ÿ0.37 < x < ÿ0.60 ÿ0.28 < x < ÿ1.16 ÿ0.35 < x < ÿ0.78 ÿ0.51 < x < ÿ0.75 ÿ0.66 < x < ÿ0.93 ÿ0.34 < x < ÿ1.03 ÿ0.55 < x < ÿ0.92 ÿ0.56 < x < ÿ0.74

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

264 Table 7 (continued ) Locality

Year

Month

R2

Kirkùy/ Norway

1997

May

0.97

7

1997 1997 1997 1997 1997

June August September All May

0.80 0.88 0.62 0.70 0.79

1997 1997

September All

0.83 0.80

érnekupa/ Norway

Degrees of freedom

F-value

p

Z

95% confidence interval

227

<0.0001

ÿ0.40

ÿ0.35 < x < ÿ0.45

7 7 7 34 7

29 52 11 81 26

0.0010 0.0002 0.0118 <0.0001 0.0013

ÿ0.36 ÿ0.77 ÿ0.40 ÿ0.48 ÿ0.46

ÿ0.23 < x < ÿ0.50 ÿ0.56 < x < ÿ0.98 ÿ0.17 < x < ÿ0.62 ÿ0.38 < x < ÿ0.59 ÿ0.29 < x < ÿ0.64

7 16

35 66

0.0006 <0.0001

ÿ0.41 ÿ0.44

ÿ0.27 < x < ÿ0.54 ÿ0.33 < x < ÿ0.54

The collected dataset from VendelsoÈfjorden/ Horta in 1988 originated from a test fishing.

escapement in this area, that is, the number of eels that might be expected to migrate from the coast in order to spawn in the Sargasso Sea. 5. Discussion Eel ®shery catches on the Swedish west coast have remained stable or even increased while at the same time in the Baltic Sea, ®shery of silver eel has been substantially reduced (e.g. SvedaÈng, 1996a). The reasons for this difference are undoubtedly related to the fact that eel stocks on the west coast bene®t from their geographical location as this coastal area is nearer the major sea currents, bringing glass eels from the Atlantic to Europe, unlike the Baltic Sea. Records of daily catches as well as the long-term test ®shing support the view that recruitment to the west coast ®shery is stable, since there is no evidence that eel yields have been maintained as a result of increased effort. Also, the great similarities in size and age distribution in all the investigated localities support the theory that variations in glass eel recruitment have moderated during the years prior to the ®sh being recruited to the ®shery. Since glass eel recruitment has been considerably reduced over the past 15 years on the west coast (SvedaÈng, 1996a), density-dependent regulation at the elver and yellow eel stages could be one possible moderating factor. It can also be concluded that eel ®shery on the Swedish west coast still seems to rely on a good resource base. The magnitude of the estimate of the total instantaneous rate of mortality from the Koster Islands was

similar to other estimates of natural mortality in marine species. Vùllestad (1986, 1988) studied the instantaneous rate of both total and ®shing mortality in a yellow eel stock in the Oslofjord area, that is in Norwegian coastal waters adjacent to the Swedish west coast, by catch-curve analysis and using the mark-recapture technique. The total rate of instantaneous mortality was estimated to be 0.48 (the corrected value in Vùllestad, 1988), thus similar to the total mortality rates in the Norwegian area included in this study. However, the ®shing mortality was estimated to be very low, varying between 0.02 and 0.08, thus implying a very high rate of natural mortality, in contrast to the estimate obtained in this study. This difference could be due to an under-estimation of the ®shing mortality in the Oslo fjord area, perhaps due to failures in the mark-recapture technique. A more reliable estimate of natural mortality has been obtained from a closed freshwater system in southern Norway (Vùllestad and Jonsson, 1988), showing between 1975 and 1987 a mean mortality rate from the elver stage to the silver eel stage of 0.17, which indicates a much lower rate of natural mortality in comparison with the results of this study, as the estimate from the freshwater system included the ®rst years of the yellow eel stage. One reason for the lower natural mortality rate in the freshwater system could be that the survival rate in temperate climates is, in general, lower for ®sh in marine habitats, due to the greater numbers of predators at sea (Gross et al., 1988). Given the fact that the catches in the professional eel ®shery were dominated by small-sized eels and

H. SvedaÈng / Fisheries Research 40 (1999) 251±265

that the ®shing mortality rate was high, implying socalled growth over-®shing and a low escapement rate, it is doubtful whether this natural resource is being wisely managed from a biological point of view. An increase of the present minimum size limit of 370 mm in total length, giving a higher escapement rate, which, if applied generally, would perhaps be an important adjustment in terms of protecting overall recruitment of the European eel (cf SjoÈstrand, 1996; Moriarty and Dekker, 1997). However, a large number of eels caught in the lenght interval between 370 and 490 mm (Holmgren and WickstroÈm, 1988) are used for stocking purposes in inland waters as well as along the Swedish Baltic coast. Thus, if the escapement rate for stocked eels is at a satisfactory level, which has to be further investigated (cf SvedaÈng, 1996a), could the intensive eel ®shing on the Swedish west coast have less negative effects on stock recruitment than estimated in this study. Acknowledgements I would like to thank Kurt Torildsson for carrying out the tedious work of measuring all the eels. References Bergersen, R., Klemetsen, A., 1988. Freshwater eel Anguilla anguilla (L.) from north Norway, with emphasis on occurrence, food, age and downstream migration. Nordic J. Freshwater Res. 64, 54±66. Erichsen, L., 1976. Statistik oÈver aÊlyngeluppsamling i svenska vattendrag (Statistics on eel collecting in Swedish rivers). Information fraÊn SoÈtvattenslaboratoriet, Drottningholm 8, (in Swedish). Gross, M.R., Coleman, R.M., McDowall, R.M., 1988. Aquatic productivity and the evolution of diadromous fish migration. Science 239, 1291±1293. HagstroÈm, O., WickstroÈm, H., 1990. Immigration of young eels to the Skagerrak±Kattegat area 1900±1989. Int. Revue Ges. Hydrobiol 6, 707±716. Holmgren, K., Mosegaard, H., 1996. Plasticity in growth of indoor reared European eel. Nordic J. Freshwater Res. 72, 63±70. Holmgren, K., WickstroÈm, H., 1988. The quality of Swedish yellow eels used for stocking in 1987 ± A study of sex, size, age and wounds. Information fraÊn SoÈtvattenslaboratoriet, Drottningholm 8, (in Swedish with English summary). Lara, M.J., 1994. Catch statistics, capture methods, size, and development stages of glass eels in Asturias (northwestern Spain). Bull. Sea Fish. Inst. (Gdynia) 1(131), 31±39.

265

Moriarty, C., 1990. European catches of elver of 1928±1988. Int. Revue ges. Hydrobiol. 75, 701±706. Moriarty, C., Dekker, W., 1997. Management of the European eel. Fisheries Bull., Dublin 15, 110. È ber die Aalwanderung im Baltichen Meer auf MaÈaÈr, A., 1947. U Grund der Wanderaalmarkierungsversuche im Finnischen and Livischen Meerbusen i. d. J. 1937±1939. Medde. Stat. Unders. FoÈrsoÈksanst. SoÈtvattenfisket 27, 1±56. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bulletin of the Fisheries Research Board of Canada, p. 191. Rossi, R., Villani, P., 1980. A biological analysis of eel catches, Anguilla anguilla L., from the lagoons of Lesina and Varano, Italy. J. Fish. Biol. 16, 413±423. SCB, Statistiska centralbyraÊn. Fiske. Statistiska meddelanden 1914±1995. (Fishery. Statistical notes) (in Swedish). SjoÈstrand, B., 1996. Where have all the eels gone and does anyone care? ICES Information 28, p. 9. SvedaÈng, H., 1996a. The development of the eel (Anguilla anguilla (L.)) stock in the Baltic Sea: An analysis of catch and recruitment statistics. Bull. Sea Fish. Inst. (Gdynia) 1(139), 29±41. SvedaÈng, H., 1996b. The dispersal of the swimbladder nematode Anguillicola crassus in eel in Swedish coastal waters. Kustrapport, 4, (in Swedish with English Summary). SvedaÈng, H., Neuman, E., WickstroÈm, H., 1996. Maturation patterns in female European eel: Age and size at the silver eel stage. J. Fish Biol. 48, 342±351. SvaÈrdson, G., 1976. The decline of the Baltic eel population. Drottningholm: Report from the Institute of Freshwater. Research 55, 136±143. Thoresson, G., 1976. Projekthandbok foÈr faÈltundersoÈkningar. (Manual for field studies). Swedish Environmental Protection È regrund, December 1976 (in Swedish). Board. O Thoresson, G., 1992. Handbok foÈr kustundersoÈkningar. Recipientkontroll. (Manual for coastal studies. Monitoring of recipients). Swedish National Board of Fisheries, Kustrapport 4 (in Swedish). Thoresson, G., 1996. Guidelines for coastal fish monitoring. Swedish National Board of Fisheries, Kustrapport 2. Vùllestad, L.A., 1986. Growth and production of female yellow eels (Anguilla anguilla L.) from brackish water in Norway. Vie Milieu 36, 267±271. Vùllestad, L.A., 1988. Tagging experiments with yellow eel Anguilla anguilla L. in brackish water in Norway. Sarsia 73, 157±161. Vùllestad, L.A., Jonsson, B., 1986. Life-history characteristics of the European eel Anguilla anguilla in the Imsa River. Norway. Trans. Am. Fish. Soc. 115, 864±871. Vùllestad, L.A., Jonsson, B., 1988. A 13-year study of the population dynamics and growth of the European eel Anguilla anguilla in a Norwegian river; evidence for density-dependent mortality, and a model for predicting yield. J. Anim. Ecol. 57, 983±997. Vùllestad, L.A., Lecomte-Finiger, R., Steinmetz, B., 1988. Age determination of Anguilla anguilla and related species. European Inland Fisheries Advisory Commission Occasional Publications, vol. 21, pp. 1±28.