Sprat Stock of the Open Soa in the Southern Baltic. Jan Elwertowski

Transcription

1 Sprat Stock of the Open Soa in the Southern Baltic by Jan Elwertowski Since many years, When the season of winter catches is over 9 tords the end of April the fishermen of the Polish cutter fisheries coence the exploitation of very large concontrations of sprat, present both in the Bay of Gdansk and in the area of the Gdansk Basin in the open sea, at a distance of 5 to 8 sea miles from fishing ports. During the top of spring season these concentrations sprcad in the form of wide stripe from Gdansk Bay towards N.vr. to the regions situated in the eastern part of Slupsk Furro and to tho slopes of Middle Bank. stable slopes lliga. t appears from Soviet research information that at the same time concentrations of sprat are also present in the Gotland Basin and of Hoburgs Bank, as wel as in the region off the entrance to the large and over the Gulf of The most intensive fishing is carried out by Polish fishermen in the region situated in tho east passage botcen 3upsk Furrow and the northern area of Gdansk Basin. The fishing is performed both with trawls and with two-boat sprat trawls (pnrejas), bottom and mid-water ones, and in the evaluation to comparab1e indicos evident and essential differences appear for these two types of goar. n general, vory high yields are obtained, and thus for oxample in y 1964 the highost recordod landing from a 3 hour trawling amountod to 36 tons of sprat. The total quantity landed is however, 1imitüd by the availab1e capacity of fishing ports to handle and process these amounts of sprat. For thes3 rensons tho landings of sprat oscillate round the level of 1-11 thousand tons, whereas in particularly favourable sensons they can rüach 15 thousnnd tons. Such a 1anding is cumulated for the period of 6-8 weeks in April and May, und partly in June. The catches are composed pnrtly by the sprat in running stage, in majority by maturing sprat. Hence spring concentrations may be defined as spavming ones. The investigations on length and age-composition of sprat shoa1s, captured in the open sea, were cocnced in 1955; of a more regular and continuous character have these investigations been sinco n view of the fact that similar investigations were carried out in the coastal fishing grounds of tho Bay of Gdansk, tho comparison of the shoals exploited by the fisheries in both regions became possible. captured in Gdansk Bay and in the open area of Gdansk Basin (in the eastern part n Figure 1 is shown the mean length and ago-composition of sprat shoals, of Slupsk Furrow for the period of ). n spite of tho fact that the period, for which tho meen composition was calculated, was not particulnrly typical (owing to the prosence of a vory rich generation of 1959 and the series of three poor generations of the ycars 196, 1961 and 1962) the spccific charactoristics of sprat concentrations in euch of the regions invcstigatcd bear permanent and regular character, recurring cvery yenr. 1. The fish, concentrated during the spaning period of spring in the coastal fishing grounds of the Bay of Gdansk, are mainly young yenr-classes of smaller size. At thc same time, in tlle area of Gdansk Basin in the open sea thc basis of concentrations and of spawning stock ke larger fish, belonging to older year-c1asses. 2. The compositions, shov.n in Figuro1,rc1ate to spaving shoals of sprat and therefore illustrate the process of rccruitment into thc spavming stock, concentratod in the coastal ters of Gdansk Bay and in thc open sea. n the Bay of Gdansk this process begins in the first year and usually cnds in the third year of the life of sprat, be1nging to each generation, though it is as a rule most intensive in thc second year of life, loss often in the third year. n Figure 1 the peaks of intensity of rocruitment into the spa\ing stock in both regions are delaycd by one year, which is due, as already mcntioncd, to a particular situation, which took placo in th5 sprat stock Dl the ycars (very nicn generation follocd by three successive poor gcncrations orc detailed

2 ( : \ 2 - \ analysis of the proeess of reeruitment and simllltaneous nfter wintoring is given in Table l. sogregation of thc stock Tho comparntive dtl have been prepared for three years (1955, 1959 and 1961) when three rieh generations, born in the years 1953, 1957 and 1959, appeared in the stock as two-yonrs-old fish. The changes of frequency of these generations nnd tho genorntions following them in their consecutive life phascs: vlintoring phase and spawning phnse - are represented in Table 1. n relation to initial stage (ntering phase) these chnnges must be attributüd to the process of recruitment into the spawning stock according to the spavming grounds, i.c. according to the depth and the distance of the spawning ground from the coast. Tho ches consist of the diminishing proportion of one-and two-years-old fish und of the respoctive increase in tho number of older fish in the ratio. Both these processes take placo in n loss evident form in the Bay of Gdansk, whereas they are distinctly pronounccd in thc fishing grounds of thc open sea, which is, in our opinion, the result of a cumulated effect of two natural segrogating factors, that is a physiological factor - sexual maturity, and a physical factor - the depth of the regions of spa\vning und their distance from the coast. The effect of tho latter factor upon particular generation in various years of the fish life is given in Figure 2. t may be seen from this Figure that among younger yearclasses (two-and threo-years-old fish) most of the fast-growing fish of the nctually largest size appear already in the spawning stock in thc open soa and hence the right portions of tho curves, ropresenting the composition of this stock are shiftcd towards greater lengths. n respect of the oldor year-classes (four-and five-years-old), which complotoly or almost completely started a deep-ter manner of life in the open waters of Gdansk Basin, the curves of the length distribution of the investigated generations aro identical both for the Gdansk Bay and for the open sea. Conclusions 1. n the months of spring (spawning phase) in the fishing grounds of the open sea older und larger sprnt appcar comparcd with concentrations of this species, exploited in the coastal fishing grounds of Gdansk Bay; 2. The recruitment into tho spawning stnck of the open soa begins in thc socond-third year of fish life, ends in its fifth year of life, whercby thc most intensive recruitment takes place in the fourth year of life of each generation. 3. The most early and most intensive migration from coastal fishing grounds into tho spavming stock of the open sea is obsorvcd in tho two-and thrcc-yearsold fish (rccruits) of tho fastest growth

3 .e Tab1e 1. Length- and Age-Composition cf the Sprat Catches in the Bay of Gdansk und in the open Sea (eastern part of Slupsk Furrow) in the years 1955, 1959 und Phase of T.h-(cm.) - Aft,tL U earu- Y R life 6 i Tl2T total 1 i Tot.1 ear:::1 egons cyc1e frequsncy in % -. Ba of! ;! i _llsj:.. inorin -=_l 116,... 33t3l22_!,:_ : _Ll3_1_--" 9 OOO _ (cate;n spawning \ PSa1r k F ) ffi ups i LL_:,: ll-2 _j O _ Bay of Gdansk :::pawning ' t '---' (e;rea opawning , 13j part cf Slupsk F.) Bay of Gdansk wintering &r-' Gdansk spawning..._ Open s ea (eastern part of Slupsk F.) spawning f _: _ r::-----_5 o _O " l _ i l; l: 15 n brackets: the populations observed in the catches during particu1ar yeurs

4 ( Figuro 1. Average 1ength and age-composition of the sprat catches in the Bay of Gdansk and in the open sea taken during spring seasons of thc pcriod Bay cf Gdansk ! cm, t- _ r_4-+_5-\-_6-tJ years J '-- io- B. Open sea (castern part cf Slupsk Furrow) 5 4 % j--l 8 C. Differonces bctween Bay cf Gdansk and open sea + ++-j ++ --t-+, --+.'-'-..-=-:r-t e cm years

5 d 6cf-: 4 '!oo e 6 1\ -5-. t Figurc 2. Length {stribution of particu1ar oenerntions caught fu t.. T Bny of Gdansk ELnd in tho opon sea (eastern part of tho S±upsk Furrow [1)./2 yoars- ld----- in tao sprin soasons cf tho years 1955, 1959, >t +: + generations e!co d) 5 ars 17 i } : :/ 1954 f. / -+- -;.. -rf -\- f -\- j) ",,;j ---" ;' P r--, Bay of Gdansk ; Opon sea (eastom part / \ / of S±upsk Furrow)