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Not to be cited without prior reference to the author International Council for The Exploration of the Sea
C. M. 2000/N:06 Spatial and Temporal Patterns in Recruitment Processes
Causes of spawning pattern variability of anchovy and hake on the Patagonian shelf by Martin D. Ehrlich1, Patricia Martos2, Adrián Madirolas3 and Ramiro P. Sánchez4 1 : National Institut for Fisheries Research and Development, INIDEP, Escollera Norte, RA-7600 Mar del Plata, Argentina and University of Buenos Aires. e-mail:
[email protected]. 2 : National University of Mar del Plata and INIDEP, e-mail:
[email protected] 3 : INIDEP e-mail:
[email protected]. 4 : INIDEP and National University of Mar del Plata. e-mail:
[email protected]
ABSTRACT The hake and anchovy are the most important demersal and pelagic resources on the Argentine shelf. Both species have a common geographical range, as they are trophically linked: the anchovy is one of the main food items of the hake during their adult stages. While the hake is heavily exploited, with clear signs of overfishing, including a seriously reduced spawning biomass, the anchovy is relatively unexploited. Traditionally both species have shared the same reproductive scenarios off Patagonia characterised by tidal mixing fronts acting as larval retention areas. This paper reviews the spawning and nursery ground of both species since early 1970’s. The most striking result is the gradual disconnection of the hake from the traditional spawning grounds, which are still occupied by the anchovy. The typical hake spawning concentrations are now dispersed and located offshore, while the anchovy maintains the classical pattern in close association with the frontal system. An analysis of the hydrographical conditions prevailing in the spawning grounds shows no significant changes that could account for this anomalous hake behaviour. The implications of these changes on recruitment and population dynamics of both species are discussed in this paper. Keywords: anchovy, hake, spawning, fronts, patagonian shelf INTRODUCTION Hake (Merluccius hubbsi) and anchovy (Engraulis anchoita) are the most important demersal and pelagic fishing resources on the Argentine shelf. Both species share an ample distribution on the Argentine shelf as it is appraised in figure 1. They have an important trophic link since anchovy is the main prey of adult hake. The hake is now overfished with catches that widely exceed the maximum permissible catches (Pérez et al., 2000).
2 34º S
34º S
50 m
50 m
35º
35º
Río de la Plata
36º
36º
Fall-Winter
37º
37º
Fall-Winter
38º
38º 39º
39º
El Rincón
El Rincón
40º
40º
41º
41º
42º
42º
Spring-Summer
Península Valdés
43º
43º
Isla Escondida
Spring-Summer
Isla Escondida
44º
44º 45º
45º
Engraulis anchoita
47º
47º
Main spawning places
Adult distribution
Main spawning places 49º 50º
200 m
50º
200 m
48º
48º 49º
Islas Malvinas
46º
Merluccius hubbsi
46º
Adult distribution
51º
51º
52º
52º 100 m
100 m
53º
53º
54º
1000 m
54º
1000 m
55º
55º 56º 70º W 68º
66º
64º
62º
60º
58º
56º
54º
52º
50º
56º 70º W 68º
66º
64º
62º
60º
58º
56º
54º
52º
50º
Figure 1. Adult distribution and main spawning places of anchovy (Engraulis anchoita) and hake (Merluccius hubbsi) On the contrary, the fishing of the anchovy is moderate and with constant annual catches of approximately 10000 t. Figure 2 shows the annual landings of both species according to data of the Argentine Government.
700000
Hake Anchovy
Landings (t)
600000 500000 400000 300000 200000 100000 0 1965
1970
1975
1980
1985
1990
1995
Year
Figure 2. Argentine landings of anchovy and hake (all stocks) from 1965 to 1999 Both species are partial spawners and ubiquitous for the spawning and nursery sites. In figure 1 the main spawning areas throughout the year are presented. It is important to emphasise that they share the spawning area, particularly in spring-summer in the northpatagonic coastal zone between the 43º and 45º S, to about 100 m depth. In
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these latitudes like in the rest of the Argentine shelf, the platform is characterised by its smooth slope starting with depths near the coast of 20 m. This area stands out by a special hydrographic regime characterised by the presence of tidal fronts. These fronts generated in spring and summer form a transition between vertically well mixed waters by tidal action and stratified waters where the surface heating is dominant. This frontal zone offers conditions of production, retention and feeding necessary for survival the first stages of several fish species that live in the area, particularly the anchovy and the hake. Due to its fishing and hydrographic importance this zone, which is the main spawning area for the patagonian hake stock, has been studied intensely since the seventies. This paper presents the annual variations of the distribution and abundance of the spawning adults of anchovy and hake in comparative form with the eggs densities in the plankton. The analysis takes place considering the interannual variability in the position of the fronts and the interannual variations of the sea temperature. Finally, the variations in the spawning patterns of these species, considering the effects of different fishing intensity over these resources are presented. MATERIAL AND METHODS To estimate the distribution and abundance of anchovy and hake eggs, eight cruises always made in December from 1973 to 1999 have been considered, with a total of approximately 119 stations for hake and 354 stations for anchovy. The ichthyoplankton samples were obtained with the normal techniques for these tasks and the densities calculated as eggs by 10 m2 of sea surface. To calculate the average densities, the delta distribution was used. To obtain the egg index by cruise or year the value corresponding to the station with greater egg density were used. The spatial egg density distributions were represented by isoareal charts using the inverse distance in a power gridding method with a power factor of 10. The abundance of spawning adults of the hake and anchovy have been obtained, since 1988 by acoustic methods in the same campaigns used for the estimation of the density and abundance of eggs. Acoustic data were obtained by means of a SIMRAD EK400 scientific echosounder and a SIMRAD QD echointegrator in the 1988 survey. Since the 1994 survey, a SIMRAD EK500 echosounder and a graphical workstation with SIMRAD BI500 post-processing software was employed. The data processing method was echointegration for which different depth strata were defined in order to facilitate the separation of the scattering values of the different species. Results are presented in units of column backscattering strength (sa) normalized per squared nautical mile. The hydroacoustic evaluations were completed with fishing hauls to know the sex ratio, size distribution and reproductive condition of the specimens. For the analysis of the oceanographic conditions of the area, data obtained from the same cruises for ichthyoplankton have been used. A total of 328 CTD profiles obtained in December have been analyzed since 1984. The stratification of the water column were quantified through the stability parameter φ (Simpson and Bowers, 1981). The location of the fronts in the different years was defined using a critical value of 40 J/m3 (Martos and Sánchez, 1997), limit between homogeneous and stratified waters. From the previous knowledge and according to the characteristics of the vertical
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structure of the thermal field, the area was divided in 3 sectors. The sectors were the following ones: 1) homogeneous, defined where the temperature difference between surface and bottom was smaller or equal to 1°C, 2) frontal, where the temperature difference is between 1 and 3°C and 3) stratified, where the temperature differences were greater to 3°C. In Figure 3 the results of the classification of the data in the different sectors can be observed, and also the average position of the front in the mentioned period. According to this grouping, temporary series of average temperature for the homogeneous sector and surface and bottom for the stratified sector were made. To compare the variations of temperature between the different years, a non-parametric statistical analysis was made. For the global comparison among the different years, a Kruskal-Wallis test was used; whereas for the comparison of the years in a paired form, the Dunn test was used.
40°
S
Stratified Sector Frontal Sector Homogeneous Sector
41°
Mean Frontal Position 42°
43°
44°
45°
50 m 68°W
67°
100 m 66°
65°
64°
63°
62°
61°
46° 60°
Figure 3. Distribution of data in homogeneus, stratified and frontal sector. The line indicates the mean frontal position in the analized period RESULTS Oceanographic characteristics The position of the frontal area in the different analysed years, established from the critical value of stability, is observed in figure 4. The front displays a parallel direction to the bathymetry. The estimated position of the fronts in the area of Valdés Peninsula oscillated between 80 to 120 km of the coast. To the south in the zone of Isla Escondida the distance of the front to the coast is smaller and oscillates between 20 to 100 km. During December 1995, 1996 and 1998 the front appeared very near the coast in the area of Isla Escondida. On the other hand, in 1994 the front was in its maximum distance offshore.
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40° S
41°
1998 1993 1994 1995 1999
1996
42° Valdés Peninsula
1988
1984
1986
43°
Escondida Island
44°
45°
50 m
68° W
67°
100 m
66°
65°
64°
63°
62°
61°
46° 60°
Figure 4. Frontal position determined by a critical value of 40 J/m3 of stability parameter (Simpson and Bowers, 1981) in different years of the analized period The analysis of the characteristics of the thermal field in the different sectors defined by the front position showed the existence of interannual variations. The average temperature in all the analysed period for the homogeneous sector was of 12.94 °C, with a minimum of 11.31 °C in 1995 and a maximum of 15.08 °C in 1986. The temperature values, averages and deviations per year for the homogeneus sector appear in Table 1. Table 1. Temperature statistics in the homogeneus sector Year
Min-Max. (°C)
1984
Mean
Sd
N
11.6
0
1
1986
12.14-13.84
13.17
0.91
3
1988
11.68-16.63
12.32
0.70
6
1993
11.43-13.18
12.22
0.82
5
1994
11.52-13.5
12.59
0.58
16
1995
11.31-13.88
12.47
0.69
18
1996
11.58-15.08
13.65
1.14
10
1998
13.08-14.99
14.15
0.57
15
1999
12.24-13.83
12.94
0.41
13
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The average temperatures per year oscillated between a minimum of 12.22 °C (1993) and a maximum of 14.15 °C (1998) (Figure 5) without considering 1984 with only one value. The statistical analysis of the interannual variations of the average temperature made in global form shows highly significant differences.
.2 12 .
9
.6 14 .3
6 12
12
.2 12 .
3 12 .
.6
13
.2 13
14 11
Temperature (°C)
16
12 10 8
1999
1997
1995
1993
1991
1989
1987
1985
1983
Year
Figure 5. Interanual variation of the mean temperature at the homogeneous sector for the period analized. The bars indicate the standard deviation The comparison of paired years only shows significant differences between 1995-1996 and 1993-1998. In Figure 6 thermal anomalies in every year with respect to the total mean can be observed. The maximum positive anomaly (1.20 ºC) was in 1998 and the negative minimum (0.71 ºC) in 1993.
Thermal anomalies (ºC)
1,5 1 0,5 0 1983
1985
1987
1989
1991
1993
1995
1997
1999
-0,5 -1 -1,5
Figure 6. Thermal anomalies for the period analized on the homogeneous sector In the stratified sector, the fields of surface and bottom temperature were analysed separately. The surface field shows an absolute average value of 14.44 °C, with a minimum of 12.61 °C in 1995 and a maximum of 16.77 °C in 1998 (Table 2). The average values per year oscillated between a maximum of 15.48 °C (1996 and 1998) and a minimum of 13.48 °C (1995) (Figure 7). The interannual variations analysed statistically show highly significant variations. The comparison of paired years shows significant differences between 1988-1993, 1993-1994 and 1993-1998.
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Table 2. Surface temperature statistics in the stratified sector Year
Min-Max. (°C)
Mean
1984
Sd
N
13.44
1
13.66-15.36
14.40
0.53
23
1988
13.54-15.06
14.15
0.35
17
1993
12.91-14.27
13.77
0.36
27
1994
13.09-15.68
14.42
0.62
27
1995
12.61-14.44
13.48
0.47
23
1996
14.34-16.13
15.48
0.47
34
1998
14.97-16.77
15.48
0.46
23
1999
13.8-14.16
13.99
0.14
10
15
.5
.0
15 8. 5
14
9.
8.
1
10
.6 5
6 9.
6 9.
4 9.
3 9.
1995
10 8
10
Surface
12
.1
4
.5 13
13
14
.8 14 .
.1
.4 14
13 .
4
16 14
1993
Temperature (°C)
18
.5
1986
Bottom
6
1999
1997
1991
1989
1987
1985
1983
Year
Figure 7. Interanual variation of the surface and bottom mean temperature at the stratified sector for the period analized. The bars indicate the standard deviation
The average bottom temperature of the stratified sector was 9.36 °C, with a maximum of 12.34 °C and a minimum of 6.59 °C both occurred in 1996 (Table 3).
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The annual average values show a maximum of 10.64 °C in 1998 and a minimum of 8.09 °C in 1993 (Figure 7). The statistical tests show the existence of highly significant interannual variations. In the paired comparison between years, the greater differences were 1993-1994, 1994-1995, 1995-1996, 1998-1999 and between the coldest (1995) and the warmest (1998). In the Figures 8 and 9, the thermal anomalies per year for the surface and bottom fields of the stratified sector are presented.
Table 3. Bottom temperature statistics in the stratified sector Year
Min-Max. (°C)
1984
Mean
Sd
N
9.29
1
1986
7.24-11.14
9.4
1.85
23
1988
7.93-10.63
9.56
0.87
17
1993
6.83-9.83
8.09
0.81
27
1994
7.2-11.47
9.63
1.31
27
1995
6.77-10.34
8.49
0.98
23
1996
6.59-12.34
9.54
1.66
34
1998
8.19-12.10
10.64
1.03
23
1999
8.97-10.95
10.14
0.63
10
Thermal anomalies (ºC)
1,5 1 0,5 0 1983
1985
1987
1989
1991
1993
1995
1997
1999
-0,5 -1 -1,5
Figure 8. Surface thermal anomalies for the period analized on the stratified sector
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Thermal anomalies (ºC)
1,5 1 0,5 0 1983
1985
1987
1989
1991
1993
1995
1997
1999
-0,5 -1 -1,5
Figure 9. Bottom thermal anomalies for the period analized on the stratified sector Biological characteristics Due to the intense fishing activity on the hake, the changes produced in the spawning group of Isla Escondida are very important. Between 1988 and 1999 through cruises OB-08/88 and OB-09/99 it was observed, according to the hydroacoustic estimation, that the reproductive biomass has decreased 90 % (Madirolas & Castro Machado, 2000). The analysis of the catches of the complementary hauls to the acoustic evaluation, in the same campaigns, displays that the average density of females larger than 50 cm, that correspond to age 6 or older, has decreased 81 %. The mean egg production originating by females larger than 50 cm has decreased 79 %.
2
(eggs/10m )
Max. Egg Density
Variations in the spawning intensity of the hake and anchovy are shown in figure 10. The figure shows a noticeable and constant decrease of the maximum densities of the hake eggs of the order of 87000 eggs/10m2 in 1973 to 5100 eggs/10m2 in 1999. However, in the last years there have been anchovy fluctuations that have oscillated between 76000 to 36000 eggs/10m2 without displaying a clear tendency. Figure 11 shows the values of the indices of stability in the stations with greater density of eggs. It is observed that for the anchovy they fluctuate without a clear tendency, whereas for the hake it indicates a displacement of frontal zone (40 J/m3) to the stratified zone (> 100 J/m3). 100000,00 80000,00 60000,00 40000,00 20000,00 0,00 1970 1975 1980 1985 1990 1995 2000 Year Hake
Anchovy
Figure 10. Anchovy and hake maximum egg density from 1973 to 1999
3
Stability (J/m )
10
300 250 200 150 100 50 0
Anchovy Hake
1985
1990
1995
2000
Year
Figure 11. Stability in the stations of maximal hake and anchovy egg density from 1986 to 1999
Variations in the position of summer spawning places of hake and anchovy are analysed comparing the distribution of spawning adults and the egg presence in plankton. With the purpose of observing the most outstanding changes, the extreme patterns from1988 and 1999 are presented.
43º
December 1988 Spawning adults
S
43°
S
December 1988 Egg Density
100000
20000
10000
sa [m2/ mn2]
10000
5000
1000
44º
2000
44°
100 2 eggs . 10 m
1000
10
100 m
500
0
50 m
66° W
100
65°
45° 64°
63°
45º 66º W
65º
64º
63º
43º
December 1999 Spawning adults
S 43°
December 1999 Egg Density
20000
S 100000
10000
10000 5000
sa [m2/ mn2]
44º
1000
100 1000
2 eggs . 10 m 10 500
50 m
100 m 0
100
66° 45º 66º
W
65º
64º
44°
2000
W
65°
64°
63°
45° 62°
63º
Figure 12. Distribution and abundance of anchovy spawning adults and egg density in December 1988 and 1999
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In figure 12 the distribution of anchovy spawning adults and eggs in 1988 and 1999 is shown. Both groups share the homogenous zone well as the stratified one. Between these two situations separated by 11 years, important changes in the spawning pattern of the anchovy are not registered. There is a greater expansion in the distribution and abundance of eggs in 1999 than in 1988. The hake spawning pattern has changed substantially between 1988 and 1999 (Figure 13). In 1988 the greater concentrations of spawning adults and eggs are located in areas next to the coast corresponding to the homogenous and frontal zone. In 1999, the density of spawning adults has been strongly reduced and futhermore an offshore displacement towards the stratified zone is observed. The egg densities have also been reduced and follow a concordant pattern with the adults.
43º
S
December 1988 Spawning adults
-43°
December 1988 Egg Density
eggs . 10 m
2
S
100000 20000
10000 10000
-44°
sa [m2/ mn2]
1000
5000
44º 100
10
2000
0
1000
100 m
50 m
66°
W
-45° 65°
64°
63°
100
45º 66º W
65º
64º
63º
43º
S
December 1999 Spawning adults
43°
December 1999 Egg Density
eggs . 10 m
S
100000
20000
sa [m2/ mn2]
2
10000
10000
1000
44°
44º 5000
100
2000
10
100 m 50 m
1000
0
45° 66° 100
W
66º
W
65°
64°
63°
45º 65º
64º
63º
Figure 13. Distribution and abundance of hake spawning adults and egg density in December 1988 and 1999
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CONCLUSIONS AND DISCUSION It is useful to explain the causes of spawning variability (environment or fishing) through the comparative approach between the two species as anchovy and hake, which share their spawning grounds in time and space, and are under different fishing pressure. From the analysis of the hydrographic characteristics of the area, considering frontal dynamics like a leading factor of a series of biological processes there are no significant environmental changes that can affect the spawning patterns. The data obtained with respect to the continuity in the front formation, its spatial fluctuations and the observed thermal anomalies result in the conclusion that the patagonian shelf is a relatively stable environment. The observed changes of temperature are within the ranks where the anchovy eggs (Sanchez & Ciechomski, 1995) and the hake eggs (Ehrlich, 1998) can develop. Throughout the studied period, the anchovy does not present greater changes in the spawning sites, only in the last years it was detected a greater density of eggs in relation to a reduction of the abundance of the hake. On the contrary, for the hake it has been observed a reduction in the spawning biomass and an offshore displacement of the habitual spawning sites. The observed displacement in the spawning area of the hake follows a different pattern from the indicated by MacCall (1990). This author states that the spawning places for different clupeoid species, with a reduced population of spawning adults, conserve their original position despite their smaller extension. Recognizing the observed hydrographic stability in the area and the absence of changes in the spawning pattern of the anchovy, the causes of the changes in the spawning pattern of the hake must be attributed to overfishing. The studied zone is a privileged place of the hake population, where high concentrations of adults take place to spawn. The overfishing there, has produced in general, the reduction of absolute reproductive biomass and a reduction of the densities of the oldest females. The removal of larger females with greater fecundity, greater size of eggs with better viability and development (Trippel et al. 1997 and Trippel, 1998) would be one of the causes. Another cause, related to the previous one, is the diminution of females with greater reproductive experience or "homing" apt to concur to the best spawning sites, leaving the youngest females with less homing capacity to spawn offshore in other new places. The homing behavior or space learning would be similar to the indicated by MacQuinn (1997) for clupeoids. The argentine hake fulfills some indicated conditions for spatial learning behavior: it is a partial spawner, it has high fecundity, the reproductive concentrations are high, with a hidrographic larval retention mechanism, and individuals are longeveous so that this explanation seems reasonable. The next question to be answered is what reproductive value or what carrying capacity have these new places of egg concentration, may be they are the product of the vagrant behavior indicated by Sinclair (1988).These places that moved offshore from the most propitious original places must surely present other environmental conditions, of feeding and predation that affect the recruitment. Finally, it is possible that the hake, so ubicuous in its spawning places, adapts and prospers in these new areas but with different recruitment capacity.
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LITERATURE CITED EHRLICH, M. D. 1998. Los primeros estadios de vida de la merluza Merluccius hubbsi Marini 1933, como aporte al conocimiento de su reclutamiento y estructura poblacional. Tesis presentada a la Facultad de Ciencias Exactas y Naturales de la UBA para optar al título de Doctor de la Universidad de Buenos Aires, 318 pp. MADIROLAS, A. & CASTRO MACHADO, F. 2000. Monitoreo acústico del área de desove de merluza (Merluccius hubbsi) en Isla Escondida. INIDEP, Inf. Técnico 20/2000, 7 pp. MacCALL, A.D. 1990. Dynamic Geography of Marine Fish Populations. Univ. Washington Press, Seattle, 163 pp. MARTOS P. & SANCHEZ R.. 1997. Caracterización Oceanográfica de regiones frontales en la Plataforma Patagónica en relación con áreas de desove y cría de la anchoita (Engraulis Anchoita). Coloquio Argentino de Oceanografía. Bahía Blanca, 4-5 de septiembre 1997. IAPSO-IADO (Conicet). McQUINN, I. H. 1997. Metapopulations and the Atlantic herring. Rev. Fish Biol. and Fish., 7: 297-329 PEREZ, M., AUBONE, A., RENZI, M., MADIROLAS, A., IRUSTA, G. & SIMONAZZI, M. 2000. Overfishing Indicators in the Hake (Merluccius hubbsi) stock South of 41ºS, Southwest Atlantic Ocean. ICES, CM 2000/V:12. (Poster) SANCHEZ, R.P. & CIECHOMSKI J. D. de 1995. Spawning and nursery grounds of pelagic fish species in the sea-shelf off Argentina and adjacent areas. Sci. Mar. 59(3-4): 455-478. SIMPSON J. H. & BOWERS, D. 1981. Models of stratification and frontal movement in shelf seas. Deep Sea Res., 28A (7): 727-738. SINCLAIR, M. 1988. Marine Populations: an essay on population regulation and speciation. Washington Press, Seattle and London, 252 pp. TRIPPEL, E. A. 1998. Egg size and Viability and Seasonal Offspring production of young Atlantic Cod. Trans. Am. Fish. Soc., 127(3):339-359. TRIPPEL, E. A., KJESBU, O. S. & SOLEMDAL, P. 1997. Effects of adult age and size structure on reproductive output in marine fishes. In: Early Life History and Recruitment in Fish Populations (CHAMBERS, R. C. & TRIPPEL, E. A. edts.), Fish and Fisheries Series 21, Chapman and Hall, London, pp. 31-62.
