International Council ICES CM 2003/R:07 for the Exploration of the Sea Theme Session: Freshwater and Diadromous Fishes in the Baltic Sea Long-term (1976-2002) changes in freshwater and diadromous fishes occurrence in the bottom layer of the southern Baltic Sea (on the basis of the Polish Young Fish Surveys) :áRG]LPLHU]*U\JLHODQG.RUGLDQ7UHOOD Sea Fisheries Institute, ul. Kollataja 1, 81-332 Gdynia, POLAND fax: +48 58 6202831, e-mail: [email protected] ABSTRACT. The main purpose of the Polish Young Fish Surveys programme was to estimate both the spatial distribution and year-classes abundance of cod, flounder, herring and sprat in the southern Baltic Sea. Freshwater and diadromous fishes occur as by-catch in these research catches. The basic aim of this paper is to assess the long-term changes in the occurrence of freshwater and diadromous fishes in hauls and in their CPUE, in the relation to nine trawling profiles at 10-m strata between depths of 10-110 m and time periods. The results of 2953 bottom research catches carried out from 1976 to 2002 were analysed. Nine freshwater fish taxa (pikeperch, perch, ruffe, bream, roach, pike, sabrefish, rainbow trout, three-spined stickleback) were recorded in the catch composition. Moreover, nine diadromous fish species (eel, whitefish, shad, allis shad, vimba, sea trout, salmon, smelt, river lamprey) were also noted. Pikeperch was the dominant species occurring in 8.4% of the hauls followed by shad and eel at 4.3 and 3.0%, respectively. Keywords: by-catch, freshwater and diadromous fishes, geographical and bathymetrical distribution, southern Baltic. INTRODUCTION According to various classifications, the freshwater and diadromous fishes of the Baltic are considered either as commercial species with an unlimited, annual catch (except salmon for which there is a quota) or as coastal fishes, some of which are migratory or euryhaline. Some species are permanent inhabitants in particular areas, while others appear periodically within a certain time period during their life or annual cycles. Some species have also been observed sporadically in given regions. Although Baltic cod, sprat, herring and flounder contribute to the bulk of the total annual catch in majority of the Baltic countries, such species as eel, trout, salmon, pikeperch, perch, bream, are the most valuable and have the highest impact on the profitability of fishing operations in the coastal zone (Draganik 1996). Coastal fishing is the main source of income for most people living near the sea. In some the Baltic Sea regions, coastal fishes like perch, pikeperch, vimba, pike, roach, ide, eel and whitefish constitute a significant part of annual catches (Draganik 1996, Erm et al. 1991, Skóra 1996, Birzaks 1997, Thoresson 1997, Nylander 1998). For example, in Estonia until the 1990s these species constituted 2-19% of the total national catch and were caught mainly in the Pärnu Bay (Erm et al. 1991, Erm 1996). The share of freshwater and diadromous fishes in Finnish commercial sea fisheries in the Baltic Sea in 1996 was 4.7% (after Nylander 1998). However, in the Szczecin Lagoon, pikeperch, perch, roach and common bream are the basic commercial species and constitute 92% of total Polish catches in this region, and this figure rises to 96% when eel is included (:\VRNLVNL The share of the species mentioned above, for example in Estonian and Latvian catches, temporarily increased during the 1980-1990 period in connection with the introduction of fishing limits for Baltic herring and the sudden decrease in the smelt stock abundance (Erm et al. 1991, Birzaks 1997, Plikshs and Aleksejevs 1998). In contrast, the abundance of pike, whitefish and smelt have decreased in the Pärnu Bay as a result of deteriorating spawning conditions. A similar situation was also observed with several freshwater species in Polish coastal waters, namely in the Puck Bay (Skóra 1993, 1996, Jackowski 2002). The primary cause of declining freshwater fish stocks in this basin was the temporary disappearance of spawning grounds, which resulted from changes in the species structure of the phytobenthos. The deterioration of the southern Baltic environment, including Polish coastal waters and the damming of rivers, caused decreased numbers of spawners in the spawning grounds of anadromous fishes (Bartel 1991, 1993). The first information on several commercial marine Baltic fish species, and to a lesser degree on freshwater and diadromous fishes, inhabiting Polish waters can be found in the SXEOLFDWLRQVRI6LHGOHFNL 'HPHO 3 F]DOVND DQG*VRZVND 0RUH recently, complex data have been made available concerning the settlement, biology and biomass of the majority of freshwater and diadromous fishes inhabiting coastal waters of south-eastern and north-HDVWHUQ %DOWLF %U\OLVND 3OLNVKV DQG $OHNVHMHYV %RURZVNL Jackowski 2002). Scientific publications on Baltic ichthyofauna present some detailed information on fish species composition and yield in particular lagoons, bays, experimental areas and in very shallow waters. Of the 57 fish species recorded in the Puck Bay over the last 80 years, only a dozen have been the subject published ichthyological papers (Jackowski 2002). Approximately 64 species of freshwater and sea fishes and three species of lamprey occur in the coastal area of the southern Baltic Sea (within the Polish EEZ), including lagoons and bays (Skóra 1996, Hesse 1998). This figure represents almost 70% of the domestic ichthyofauna. According to Draganik (1996), a greater variety of fish species is recorded in the western part of the Polish coastal zone. The Szczecin Lagoon has the highest species abundance - Skóra (1996) reported about 52 species, Garbacik-:HVRáRZVND XQSXEOLVKHGGDWD DQG'UDJDQLN DERXW DQG:\VRNLVNL DERXWIUHVKZDWHUDQGEUDFNLVKZDWHUVSHFLHV PLJUDWRU\ species and approximately 13 sea species which occur sporadically. In the materials collected by Psuty-Lipska and Garbacik-:HVRáRZVND LQ-1995, 35 fish taxa were recorded in the Szczecin Lagoon and 27 species in the Pomeranian Bay. Moreover, the frequency of occurrence of fish species in experimental hauls in the Pomeranian Bay was year dependent. Freshwater fish species such as common bream, pikeperch and perch were fished sporadically. ,QWKHFHQWUDOSDUWRIWKH3ROLVK%DOWLFFRDVWEHWZHHQWKHKDUERXUVLQ.RáREU]HJDQGàHED Heese (1998) registered a total 14 freshwater and diadromous fishes in catches. Investigations conducted by Trella (1998) in nearly the same region show that the freshwater fishes contribution in control catches was strongly connected with the location of the research profiles and was greatest at stations located near river mouths or canals which connect lakes with the sea. In the western as well as in the eastern parts of the Polish coastal zone, 60% of all the fish species are typical freshwater fish (Skóra 1996). Approximately 40 species of fish – 24 freshwater, 7 diadromous, and 9 marine – occur in the Polish part of the Vistula Lagoon; common bream, pikeperch, eel, roach, perch, smelt, sabrefish and herring are of commercial significance (Borowski 2000). Three-spined stickleback was the absolute dominant among 14 ILVK WD[D IRXQG E\ 6DSRWD DQG 6NyUD LQ WKH VKDOORZHVW ZDWHU RI WKH *XOI RI *GDVN (within the Polish EEZ). This species comprises almost 70% of the number and more than 80% of the biomass of all the fishes in this area. Anadromous fish like salmon and sea trout are widely distributed throughout the Polish EEZ. They also occur in neighbouring areas of the Baltic. However, in the Polish EEZ whitefish LQKDELWWKH*XOIRI*GDVNZKHUHWKH\DUHFDXJKWRQO\RFFDVLRQDOO\DQGWKH3RPHUDQLDQ%D\ Vimba inhabit some Polish rivers. Lamprey catches are made only in the Vistula River and did not exceed 2 tons per year (Bartel 1991, 1993). Freshwater fishes occur most frequently in relatively shallow waters e.g. in the central coast of the southern Baltic up to one km from the shore (Heese 1995). Pikeperch is the species, which migrates the farthest into the open sea and has been caught 15 km from the shore. Bream is one of the species which is most often caught in the strip of water up to 4 km from the shoreline, mainly at a distance of 200-300 m from the shore at depths of 3 m. Roach and perch 2 mainly occurred from 50 to 300 m from the shore in the vicinity of river outlets and the canals of lakes located near the sea. The description of the quantitative relationships among the Baltic freshwater and diadromous species in deeper, offshore waters and their geographical distribution according to depth and seasons is lacking in the scientific literature. The goal of this paper is to present the long-term (1976-2002) and geographic (within the Polish EEZ) variation in freshwater and diadromous fishes frequency of occurrence in the southern Baltic as well as the variations in their catch per unit effort (CPUE, in kg/h) in relation to nine trawling profiles, each 10-m strata between depths of 10-110 m and time periods. Possible relationships between the frequency of occurrence, share in catches and CPUE of the above mentioned fishes and year, quarter, depth strata and location were also analysed. MATERIALS AND METHODS The research materials were collected by the authors within the framework of the Polish Young Fish Surveys program realised by the Sea Fisheries Institute in Gdynia in the close co-operation with the ICES Working Group on Baltic International Fish Surveys. The main purpose of this programme, also known as the BITS-surveys (bottom international trawl surveys), was to estimate both the spatial distribution and year-classes abundance of cod, flounder, herring and sprat in the Baltic Sea. Freshwater and diadromous fishes also occur in these research catches as by-catch. The material originated from 2953 bottom catches sampled from November 1976 to November 2002 during the same type of research cruises conducted on vessels of similar size. One type of fishing gear was used - a P20/25 herring bottom trawl with 6-mm bar length in the codend.