Discrimination of western Baltic spring-spawning and central Baltic herring (Clupea harengus L.) based on growth vs. natural tag information

Abstract

Gröhsler, T., Oeberst, R., Schaber, M., Larson, N., and Kornilovs, G. 2013. Discrimination of western Baltic spring-spawning and central Baltic herring (Clupea harengus L.) based on growth vs. natural tag information. – ICES Journal of Marine Science, 70: 1108–1117. In the Baltic Sea, several stocks of herring (Clupea harengus L.) are surveyed and managed separately. For assessment purposes, a spatial stock separation based on ICES subdivisions is implemented. However, especially in the western Baltic, the distribution areas of two stocks, the western Baltic spring-spawning herring and the central Baltic herring, overlap. Results of regularly conducted surveys for assessment purposes indicated variable degrees of mixing of both stocks in the survey area, based on conspicuous differences in weights/lengths within certain age groups, especially in an area known for overlapping distribution of both stocks. At present, varying fractions of the central Baltic herring stock have not been taken into account during regular surveys conducted in the western Baltic, leading to possible undetected biases in assessment indices derived from these surveys. Additionally, methods otherwise applied for stock separation of Baltic herring so far are based on parameters that cannot readily be derived during regular surveys. In this paper, we present a simple and quick method to reliably allocate herring to either stock based on a separation function derived from survey-based length-at-age data, thus facilitating a more precise estimate of biomass and abundance indices from regular surveys and commercial fisheries.

Separating two herring stocks from growth data: long-term changes in survey indices for Western Baltic Spring Spawning Herring (Clupea harengus ) after application of a stock separation function

“…All years, excluding the CBH fraction from survey data, led to major changes in mean weight‐at‐age of age groups 3–6, which then showed more realistic values that lay within the range expected for WBSSH (Gröhsler et al., ). The aforementioned unexplained peak in 3‐year‐old herring as observed in 2010 vanished after the removal of CBH from survey data.…”

“…Again (cf. Gröhsler et al., ), applying the SF to survey data revealed a variable but distinct fraction of Central Baltic Herring (CBH) in parts of the western Baltic, i.e. in SD 24.…”

“…For a detailed description of analyses and methods applied, see Gröhsler et al. (). Applying the SF, an individual herring can be assigned to either WBSSH or CBH if its length (in cm) at age A M (in months) is above or below the separator length (SF, in cm):…”

“…with = length of an individual of the corresponding stock S at age A M , = mean maximum length, k S = growth parameter and t 0,S = theoretical age at zero length of the corresponding stock. Statistical analyses and comparisons of growth parameters identified for the separation function (see Gröhsler et al., ) were conducted with the statistical software program Statgraphics (Statgraphics Centurion, Version XV, StatPoint, Inc.).…”

Abundance and distribution of larval herring, Clupea harengus (Actinopterygii: Clupeiformes: Clupeidae) in the Pomeranian Bay, Baltic Sea as an indicator of spawning sites

“…Although the Pomeranian Bay is located in SD 24, and the majority of herring spawning here belongs to WBSSH, considerable mixing with CBH occurs in the area (Podolska and Horbowy 2003, Podolska et al 2006, Gröhsler et al 2013. Herring catches take place in the Pomeranian Bay during the migrations of the fish from open waters to coastal spawning areas (Popiel 1955, Strzyżewska, 1969, Krasovskaya 2002, Jørgensen et al 2005.…”