Cruise Report - «Michael Sars»

Cruise report - «Michael Sars»

Abundance estimation of Norwegian spring spawning herring at spawning grounds 20 February- 18 March 1997.

Aril Slotte and Are Dommasnes

Institute of Marine Research, PO Box 1870, N-5024 Bergen, Norway Participants

Dommasnes, Are 20.02 - 03.03 Haugsdal, Annlaug 20.02 - 03.03 K vinge, Bjarte 20.02- 14.03 Meland, Eina 03.03 - 18.03 Nilsen, Jan Henrik 03.03- 18.03 Røttingen, Jo stein 20.02 - 18.03 Slotte, Aril 20.02 - 18.03 Øvretveit, Egil 20.02 - 18.03

Methods

Acoustical data were registered with a 38 kHz SIMRAD EK 500 echosounder and echointegrator. In addition BEl, Bergen echointegrator system, was also applied in the interpretation the data. Integrator values were divided into herring and «other categories» based on the density and appearance of the registrations, target strength (TS) distribution and data from trawl catches. Based on the acoustic observationas and age composition in the trawl catches the distribution area was divided into sub areas. Conversion of integrator readings (SA) to number of herring was achieved by the following relation between target strength (TS) and total fish length (L):

TS(db) = 20.0 ·log L- 71.9

The number of individuals (N) within a certain area (A) was given by:

6 2 N = 1/a ·SA· A where 1/a = 10 · 1,23 · L-

The area A with positive integrator values was delimited after plotting the l n.m. integrator values attributed to herring along the survey lines. The mean integrator value (SA) was the arithmetic mean of all positive values within the area A. Results

The cruise was conducted during the period 20 February - 18 March 1997. The purpose was to estimate the abundance of Norwegian spring spawning herring at the spawning grounds. However, bad weather prevented the survey from progressing. Thus, the entire spawning distribution area was not covered acoustically this year. The abundance was calculated in the following areas (For information on cruise tracks, stations, integrator values and subareas, see Figures 1-5): l) Møre west/Møre east 21-28 February (Figure l) 2) Frøyabanken, Haltenbanken and Folla 4-6 March (Figure 2) 3) Træna l O March (Figure 3) 4) Stadt and Bremanger 13-14 March (Figure 3) 5) Siragrunnen, Losegrunnen, Egersund and Karlsmedgrunnen 15-16 March (Figure 4) 6) Karmøy west, Karmøy south and Bokn 16-18 March (Figure 5)

The data on abundance in the respective areas are given in Table l. The most important result of the survey was the observation of increased abundance at the southern spawning grounds around Karmøy and at Egersund/Siragrunnen, compared to earlier years. A total of about 210000 tonnes was estimated at these spawning grounds. During the period 1990-1995 the estimates were less than 30000 tonnes.

The 1992 year class dominated together with the 1991 year class in all the areas surveyed (Figure 6). However, there was a southward increase in the proportion of older herring. In addition the mean length, condition factor and maturation stage by age (1991 and 1992 year class) also increased southwards (Figure 7).

Much of the herring north of Møre had rather small gonads compared to the herring caught further south. This was partly a result of delayed maturation, but another possible explaination is reduced fecundity. Occurence of white eggs in the ovaries indicated also a high proportion of atresia. The ovaries were sampled during the cruise and the results of the south-north differences in maturation, fecundity and atresia will be available by the end of 1997. Table 1: Point estimates (from north to south) ofbiomass (1000 tonnes), spawning stock biomass (SSB) and numbers (millions) of Norwegian spring spawning herring at spawning grounds 1997. Mean length (L) in cm and mean weight C:W) in gram is added.

Area Period Biomass SSB Numbers L w Træna 10.03 41,73 26,57 310,95 27,8 133 Folla 06.03 19,56 18,26 123,77 31,4 230 Haltenbanken 05-06.03 63,01 58,83 398,76 31,4 230 Frøyabanken 05-06.03 150,39 132,95 875,49 29,4 172 Møre west 21-28.02 908,73 908,73 3988,27 31,4 230 Møre east 21-28.02 458,36 458,36 2342,94 30,3 197 Stadt 13-14.03 17,17 17,17 76,36 31,3 224 Bremanger 13-14.03 359,34 359,34 1598,05 31,3 224 Karmøy west 16-18.03 90,86 90,86 379,57 32,2 243 Karmøy south 16-18.03 3,83 3,83 15,99 32,2 243 Bokn 16-18.03 17,51 17,51 76,59 31,7 233 Egersund 15-16.03 2,71 2,71 11,87 31,9 252 Losegunnen 15-16.03 8,49 8,49 30,28 32,9 281 Sira grunnen 15-16.03 91,45 91,45 406,39 31,4 232 o

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Figure 7. Latitudinal differences in length, condition _factor and maturation among 5 and 6 year old herring ( 1992 and 1991 year dass) during the spawning period 1997.