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Theme Session on Evidence of global warming effects on zooplankton populations and communities, including larvae of benthic invertebrates and fish (Q)

ICES CM 2008/Q:07

Variability of the year-class strength of western Baltic spring spawning

herring (Rügen herring) – which factors determine the year-class strength?

R. Oeberst1, B. Klenz1, Mark Dickey-Collas2, Richard DM Nash3

1Johann Heinrich von Thünen Federal Research Institute for Rural Areas, Forests and Fisheries, Institute of Baltic Sea Fisheries, Alter Hafen Süd 2, D - 18069 Rostock, Germany 2Wageningen Institute for Marine Resources and Ecosystem Studies, P.O. Box 68, 1970 AB IJmuiden, The Netherlands 3Institute of Marine Research, PO Box 1870 Nordnes, 5817 Bergen, Norway

Abstract Estimates of the year-class strength of the Rügen herring in ICES Sub-division 22 to 24 that are based on the number of larvae which reach the length of 20 mm in the Strelasund and in the Greifswalder Bodden are used as first index of the new year-class (HAWG report 2008). The estimates are highly correlated with the acoustic estimates of age group 1 in the western Baltic Sea one and a half year later. That means that the year-class strength is mainly influenced by factors like SSB, fecundity, fertilization rate and hatching success as well as the survival of larvae smaller than 20 mm besides the hydrographical conditions which affect many of the factors mentioned before. The study analyses the development of the year-class strength from the hatching of larvae (5 – 7 mm) to the length of 20mm. Mean densities of hatched larvae (individual m-2) varied between zero catches and 4,663 larvae m-2. High densities of hatched larvae (more than 50 ind m-2) results in a strong decrease of the larvae numbers because the required density of food is not available. Therefore, the year-class increases if the distribution pattern of hatched larvae is relative homogeneous distributed in the total area without strong concentrations. The year-class strength is also influenced by the period where the herring larvae hatch. Numbers of larvae which hatched within May (week 18 to 21) explain about 85 % of the variability of the year-class index N20 together with the heterogeneity of the spatial distribution pattern of hatched larvae. Larvae which hatch earlier hit on low densities of food items, which are fast down grazed. Larvae which hatch after week 21 are strongly influenced by the high temperature which influences the organogenesis. It can be concluded that the optimum time frame for the reproduction of Rügen herring decreases by about one week if the mean water temperature increases by 1 °C due to climatic effects. Keywords: herring larvae, Rügen herring, Baltic Sea, year-class strength, reproduction success Corresponding author: Rainer Oeberst, Johann Heinrich von Thünen Federal Research Institute for Rural Areas, Forests and Fisheries, Institute of Baltic Sea Fisheries, Alter Hafen Süd 2, 18069 Rostock, Germany; Phone +49 381 8116 125; email rainer.oeberst@vti.bund.de

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 Introduction Combination of the population fecundity, the ecology of egg and estimates of the total larvae can be used for assessing the spawning stock biomass (Gunderson 1993). Internationally coordinated surveys estimate the egg production of mackerel and horse mackerel to estimate the spawning stock biomass every third year (ICES 2006). Larvae surveys in the North Sea are conducted to assess the spawning stock biomass of spring spawning herring (Gröger et al. 2001). The fluctuations of recruitment of herring stocks can be explained by the highly variable fluctuation of the mortality during the first year of life because certain relations between spawning stock and recruitment were not found (Lough et al. 1985). Growth rates and mortality of larvae are influenced by abiotic and biotic factors as well as by food supply (match/mismatch hypothesis), food competitions and predators (Fortier and Gayne 1990, Busch 1993). The quality and quantity of available food after the yolk resorption are important factors which significantly influence the survival of larvae (Fossum 1996, Werner and Blaxter 1980, Moteki et al. 2001). The concept of the ´point of no return´ was introduced by Blaxter and Hempel (1963), which is the stage after yolk resorption at which unfed larvae become too weak to start feeding. In the Baltic Sea correlation between the copepod biomass and the recruitment of herring and between the abiotic parameters and the survival of herring larvae were found (Laine et al. 2003). Weekly repeated surveys which nearly cover the total hatching season are only carried out in the Greifswalder Bodden and the Strelasund, the main spawning ground of the western Baltic herring (Biester 1989, Oeberst et al. 2008b). First surveys with repeated cruises were started in 1977 (Brielmann 1981, 1986, 1989) around the island Rügen to estimate the new year-class strength of herring based on the number of sampled herring larvae. Adapted surveys with weekly repeated cruises and with standardized design have been conducted by the Institute of Baltic Sea Fisheries in Rostock (OSF) in the Greifswalder Bodden and in the Strelasund during the main spawning season since 1992. Surveys are carried out in this area to estimate a year-class index of the Rügen spring spawning herring based on a maximum of eleven weekly repeated cruises where the density of herring larvae was sampled with a HYDOBIOS-Bongo net (net diameter: 600 mm; mesh size: 0.335 mm) at 35 fixed stations. The first cruises were conducted during the 14th week from 1992 to 1997 with an exception in 1996. Since 1998 the first cruises have been started between the 16th and 18th week due to a change in the survey design. The water temperature increased from the 14th week to the 26th week, the week of the latest cruises from 1998 to 2006 from about 5 °C to more than 20.5 °C with high variability from year to year. New year-class index was developed based on the weekly repeated cruises which takes the temperature dependent daily growth into account. The number of larvae which reach the length of 20 mm (N20) is highly correlated with the estimates of age group 1 of the same year-class based on acoustic methods (Oeberst et al. 2008b). The index is used as first estimate of the year-class by the Herring Assessment Working Group HAWG (ICES 2008b). The analyses showed that the index N20 strongly fluctuated from year to year with a minimum of 1.1 109 larvae in 1992 and a maximum of 21.0 109 larvae in 1996. The hypothesis of the study is that the variability of the index N20 can be explained by the spatial and temporal distribution of hatched larvae in combination with hydrographical data.

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Material and Methods Western Baltic herring larvae (the stock in Sub-division 22-24) have been sampled since 1979 close to their main spawning grounds in the Strelasund and in the Greifswalder Bodden (Fig. 1) (Biester 1989, Brielmann 1981, 1986, 1989, Müller and Klenz 1994, Klenz 2004). These Rügen herring larvae surveys consisted of a maximum of eleven weekly surveys. The current dataset consists of herring larvae samples from 1992 to 2007. The first cruises were conducted during the 14th week from 1992 to 1997 with an exception in 1996. Since 1998 the first cruises have been started between the 16th and 18th week due to a change in the survey design. The sampling consists of day-light only stepwise-oblique hauls with a 60 cm HYDROBIOS-Bongo (mesh size: 335 µm). The Bongo net was fitted with general HYDROBIOS flowmeters. The net was hauled at 3 knots, and the sampler taken to 1 m over the bottom. All plankton samples were fixed and preserved in 4% buffered formaldehyde-seawater solution immediately after capture. The sampling grid consisted of 35 fixed stations. For the analytical purposes the data were combined into five strata (Fig. 1). In addition, at each station a vertical profile of temperature (°C), salinity (PSU) and oxygen content (ml/l) was obtained using a CTD online or memory probe after each haul. Herring larvae were picked from the samples in the laboratory. They were counted and total length of each larva was measured to 1 mm below. Shrinkage, due to fixation was not corrected. Three subsamples of about 200 larvae were measured from samples with a high abundance (more than 1,000 larvae). All other larvae in the sample were counted and the number of larvae by length intervals was estimated based on the mean fraction of the length interval and the total number of larvae. Usually, only larvae of the outer net were used for the analyses. Sample of the inner net were only analysed if there was a problem e.g. unexplained flowmeter readings, sample spillage etc. of the outer net. The number of larvae per m² by length interval was estimated based on the volume of filtered water and the water depth of the tow (see Oeberst et al. 2008b). Mean number of larvae per m² were estimated by strata and the stratified mean of larvae per m² was estimated for the total area using the area of strata as weighting factors. The stratified mean of subsequent cruises indices in combination with estimates of mean daily growth as function of the surface temperature can be used to estimate indices of defined length limits (Oeberst et al. 2008 b). These indices present the number of larvae which reach the defined length limit. Indices of different length limits were used to describe the development of the year-class strength between the hatching of larvae and the length of 20 mm. Following indices were used N10 number of larvae which reach the length of 10 mm N15 number of larvae which reach the length of 15 mm N20 number of larvae which reach the length of 20 mm – the year-class index The index N10 estimates the number of larvae which are in the transition process after the yolk sac resorption to the active feeding and which immediately have finalized this process. Klinkhardt (1984) showed that herring larvae need about 6.5 day for the resorption of the yolk sac at 8 °C and reach a length about 9 mm. He also showed that the point of no return where herring larvae starve is about 3 to 4 days after the resorption of the yolk sac at 10 °C which corresponds with a length of about 10 mm. The analyses of the samples showed that the largest larvae with a visible yolk sac were 7 mm long after preservation in 4% buffered formaldehyde-seawater solution. The length limit of 10 mm was chosen to include yolk sac larvae within the transition phase taking

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into account that larvae which hatched with about 5 mm to 6 mm grow about 4 mm in mean within one week at temperatures of more than 15 °C. Studies have shown that the index N9 which estimates the number of larvae reach the length of 9 mm and N10 are highly correlated (r = 0.96). A further decrease of the used length limit to 8 mm or less results in an underestimation of the numbers of larvae, which are in the transition phase. Larvae which were hatched between two subsequent cruises can be larger than 8 mm after one week due to a growth of more than 3 mm during this week if the temperature is higher than 12 °C (Oeberst et al. 2008a). These larvae will not be included in the index. The index N15 describes the number of larvae which successfully survived the transition phase. The index N20 presents the number of larvae which reached the length of 20 mm. This index is highly correlated with the estimates of age group 1 of the same year-class based on acoustic methods (Oeberst et al. 2008b). The index is used as first estimate of the year-class by the Herring Assessment Working Group HAWG (ICES 2008b). The indices were estimated for each of the five strata of the total area under investigation. The relation between the same index by strata and the relations between the different indices were studied for the period 1992 to 2007 where the surveys were carried out with standard coverage and methods. In addition estimates of hydrographical conditions like temperature and salinity at the surface and the bottom as well as estimates of water currents based on wind force (data of the Federal Maritime and Hydrography Agency, Hamburg) were used to explain the variability of the development of the year-class strength from N10 to N20. The statistical analyses were carried out with Statgraphics Centurion (2005).   Results The number of hatched larvae which is given as index N10 strongly varied between 1992 and 2007 (Tab. 1) with a minimum of 7.3 109 larvae in 1992 and a maximum of 176.0 109 larvae in 1996. Large proportion of hatched larvae died during the transition after yolk resorption to active feeding. The quotient between N15 and N10 varied between 0.05 in 2007 and 0.45 in 1993. The value of 1.09 in 1999 is unrealistic. Higher survival of larvae was observed between N15 and N20. The quotient between N20 and N15 varied between 0.13 in 1993 and 0.74 in 2002. The highest densities of hatched larvae (N10) were observed during one or two cruises in the most years. Figure 2 shows the mean density of hatched larvae (larvae m-2) by strata for different years. In addition the mean daily growth of larvae is given by week. The densities of hatched larvae significantly differed between the strata (Tab. 2). Maximum densities of hatched larvae were observed in the Strelasund in all years. In some cases the larvae density in the Strelasund was ten times higher than in the other strata. The examples illustrate that the hatched larvae are heterogeneously distributed in the total area. The Strelasund is the smallest part of the area under investigation and is a narrow channel between the Greifswalder Bodden in south and the Baltic Sea in north with a minimum distance of about 2.0 km and a length of about 44 km. Therefore, a decrease of the very high density of hatched larvae is only possible by wind induced drift into the Greifswalder Bodden or into the Baltic Sea. Assuming a relative strong current of 15 cm sec-1 with optimum direction it follows that stable conditions between 3 to 4 days are necessary to transport larvae from the centre of the Strelasund into areas outside of the Strelasund. Larvae which hatched in the Greifswalder Bodden will be distributed in larger areas within short periods also during lower currents with variable directions. The highest densities of larvae which successfully finalized the transition from yolk absorption to active feeding were also observed during one or two cruises (Fig. 3). The

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maximum densities of N15 were again observed in the Strelasund in most cases, but, the densities were significantly smaller compared to N10. In contrast to this smaller decreases of larvae from N10 to N15 were observed in the Greifswalder Bodden. The significant decrease of the larvae density was observed in all years where the maximum of N10 was higher than 50 larvae m-2 in the Strelasund. The relation between N10 and N15 can be described by linear regression N15 = 5.37 + 0.232 * N10 based on 16 years with R² = 63 % (Fig. 4). The coefficient of variation increases to 73 % if only the data of the Greifswalder Bodden are used (Fig. 4). The relation between both indices based on the data of the Strelasund is dominated by the data of 1996 which results in R² of 66 % (Fig. 4). Larvae which reach the length of 20 mm (N20) are relative homogeneously distributed in the different strata (Tab. 3). In means about 50 % of N20 were captured in the southern part of the Greifswalder Bodden and about 30 % in the Strelasund. The indices N15 and N20 are highly correlated with coefficients of determination of more than 70 % (Fig. 5). Increases of the coefficient of variation of more than 15 % were observed for the Strelasund and for the total area. These increases suggest that the transition from the yolk sac to active feeding results in highly variable survival rates which is supported by the regressions between N20 and N10 where the coefficients of determination were 44 %, 47 % and 35 % for the total area, the Greifswalder Bodden and the Strelasund, respectively. The Greifswalder Bodden is the most important area for the reproduction of the Rügen herring. About 70 % of the year-class index survived in this area in mean (Tab. 3), although significantly higher densities of hatched larvae were observed in the Strelasund. Especially, the southern part of the Greifswalder Bodden (stratum 3 and 4) is more important for the successful reproduction because about 50 % of the year-class were observed in this region. Table 4 presents the proportions of hatched larvae for the periods of week 14 to 17, the selected period mentioned above and the period from week 22 to 26. It must be pointed out that a time shift of the cruises were realized from 1997 to 1998. The proportion of larvae which hatched between week 14 and 17 varied between 0.8 % and 49.2 %. On the other hand, the proportion of larvae which hatched between week 22 and 26 also varied within a wide range with a minimum of 0 % in 1993 (no sampling) and a maximum of 42.3 % (Tab. 4). Although the proportion of both periods varied within wide ranges the proportion of larvae which hatched between week 18 and 21 was relative stable with a minimum of 39.8 % and a maximum of 80.3 %. These data document that the hatching periods vary from year to year. Intensive hatching starts in some years early like in 1993, 1997 and 2003 where more than 30 % of hatched larvae were observed before week 18. In 1995, 2001 and 2005 more than 30 % of hatched larvae were observed after week 21. The proportions of hatched larvae of the periods week 14 to 17 and week 22 to 26 are negatively correlated (r = 0.51, p-value < 0.05). The early (week 14 to 17) or late (week 19 – 26) intensive hatching could neither be explained by the variability of hydrographical conditions in the Arkona Sea based on the permanent measuring of temperature and salinity in different depth layers (see reports of the of Federal Maritime and Hydrography Agency, Hamburg (www.bsh.de) nor by the acoustic estimates (ICES 2008 a, 2008b) which describe the spatial distribution of age group 2+ herring in the ICES Sub-divisions 22 to 24 in October. The results suggested that the year class strength is determined by the total number of hatched larvae and the spatial distribution patterns of hatched larvae. Extreme high densities of hatched larvae in the Strelasund significantly decreased from N10 to N15. The temporal frequency of hatched larvae further suggested that the survival of very early and late hatched larvae is relative low.

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The density of nauplii, the food of herring larvae, is low in winter with 6,000 ind m-3 in mean and reach a mean level of 15,000 ind m-3 in the Arkona Sea in April – Mai with increasing density in summer (Fennel 2001; Fennel and Neuman 2003). Postel et al. (1991) observed 200,000 ind m-3 in the Greifswalder Bodden at the beginning of June. However, the reproduction level of copepods is low in April (Fennel and Neuman 2003) so that the available low density of nauplii population is fast down grazed below the critical level by the herring larvae. Therefore, it can be assumed that the survival is relative low for larvae which hatched before the end of April. On the other hand the fertilization rate of spawned eggs decreases at temperature above 12 °C (Klinkhardt 1986). A further increase of the temperature to more than 14 °C results in a decrease of successful organogenesis (Klinkhardt 1986) and a decrease and survival these larvae after hatching. The mean temperatures during the 22th week were higher than 14 °C in the most years with increasing trend in the subsequent weeks. Therefore, it is likely that the survival of larvae which hatched after the 21th week is low. To study possible effects of the spatial and temporal heterogeneity of hatched larvae following parameter were estimated based on the available data:

• Index of N10 which was produced from the 18th to the 21th week (N10’) • Maximum mean density by strata of hatched larvae during the period mentioned

above (M_N10’) • Standard deviation of the N10 by strata during the period mentioned above

(S_N10’) • Quotient between index N10 which was produced from the 18th to the 21th week

and the total N10 • Mean surface salinity during the period used as proxy of eutrophication (Sal) • Mean surface temperature during the period (Tem)

The maximum and the standard deviation of N10 during the period mentioned above were used as proxy of the heterogeneity of the density distribution of hatched larvae. The last parameter expresses the fraction of hatched larvae within the period. The salinity of the Greifswalder Bodden is significantly determined by inflow of water from the River Peene with low salinity and high eutrophication and by inflow from the Baltic Sea which are characterized by salinity of about 8 PSU and significant lower eutrophication (Schnese 1973, Buckmann and Pfeiffer 1995). Both water bodies can be distinguished by the salinity. Multiple linear regression models have shown that the variability of the year-class index N20 can be explained by 95 % by the larvae which hatched between the 18th and 21th week. N20 = 16802 + 154 * N10’ - 431 * M_N10’ + 969 * S_N10’ + 1.115 107 * N10’/N10 – 3219 * Sal based on 16 years. The p-values of N10’, M_N10’, S_N10’ and Sal were smaller than 0.05. The p-value of N10’/N10 was 0.07. The incorporation of the surface temperature did not lead to a significant increase of the coefficient of determination. If N10’ is substituted by N10 and N10’/N10 is excluded the coefficient of determination decreases to 89 %. If the model is only based on estimates which were sampled during the 18th and 21th week (N10’, M_N10’, S_N10’, Sal) 85 % of the variability of the year-class index N20 can be explained. The survival of hatched larvae is positively influenced by water inflow from the river Peene with high eutrophication which results in a higher availability of zooplankton with high probability. Vertical water stratification in the Greifswalder Bodden is seldom (Westphal et al. 1995, Oeberst et al. 2008b).

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 Discusssion The studies showed that the large fraction of variability of the year-class strength which is estimated by the herring larvae index N20 can be explained by the number of hatched larvae and their temporal and spatial distribution. Two main factors determine the development of the larvae density from the hatching explained by the index N10 to the year-class strength. Too high densities of hatched larvae result in high mortality and a strong decrease of the larvae densities. More than 50 hatched larvae per m² require a minimum density of more than 60,000 nauplii per m³ if it is assumed that the larvae are distributed within the upper 5 meters and if the minimum level of available food density of 6,000 ind m-3 is used (Rosenthal and Hempel 1970; Kiørbe and Johansen 1986). This required density of nauplii is significant higher than the observed densities of 15,000 ind m-3 in the Arkona Sea in April – Mai and of 25,000 ind m-3 in the same area in summer (Fennel 2001; Fennel and Neuman 2003). Therefore, high mortality of hatched larvae can be expected in areas with high density of hatched larvae also if taken into account the elevated nauplii densities can be expected in the area of the shallow bank (Oder Bank), which is influenced by the nutrient loads of the Oder river or in shallow waters where the increase of the zooplankton density is significantly higher than in the deeper water of the central Arkona Sea (Fennel and Neuman 2003). Optimum concentrations of more than 200,000 ind m-3 were only observed in the estuary of the river Peene end of May/beginning of June (Postel et al. 1991). These conditions of food density are the reason that the high densities of hatched larvae in the Strelasund which is a narrow channel between the Baltic Sea and the Greifwalder Bodden significantly decreased to a low level of survived larvae (N15). On the other hand a relative small period of 4 weeks in May is the most suitable period for the successful development of hatched larvae. Larvae which hatched in April (week 14 to 17) or earlier meet densities of nauplii of less or equal to 15,000 ind m-3 (Fennel and Neumann 2003). These individuals are grazed by the hatched larvae which results in decrease of food items because the production of new nauplii starts end of April (Fennel and Neuman 2003). The beginning of the reproduction of copepods is only determined by the development of the phytoplankton bloom which starts end of March when light intensity exceeds 100 W m-2 (Moll and Stegert 2007) and is independent of the water temperature. That means that an increase of the mean water temperature due to climatic changes will not result in a change of the beginning of the production of nauplii. During May the water temperature of the Strelasund and the Greifswalder Bodden increases and reach in week 22 values of more than 14 °C. During this increase of water temperature the fertilization rate of spawned eggs decreases (Klinkhardt 1986). The further increase of the water temperature after week 22 negatively influences the organogenesis (Klinkhardt 1986) which results in decrease of the survival of eggs and hatching larvae. An increase of the mean water temperature in winter due to climatic changes can cause that the water temperature will reach the critical level of 14 °C earlier than end of May. If the mean water temperature increases around 1 °C in mean the period of successful reproduction will be reduces around one week because the water temperature increases in May with 1 °C per week in mean and will about one week earlier reach the critical values of 12 °C (fertilization) and 14 °C (organogenesis) and because the beginning of the production of nauplii will not start earlier with increasing temperature.

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 References Biester, E. 1989. The distribution of spring spawning herring larvae in coastal waters of

the German Democratic Republic. Rapp. P. v. Reun. Cons. Int. Explor. Mer, 190: 109-112.

Blaxter, J.H.S., Hempel, G. 1963. The influence of egg size on herring larvae (Clupea harengus L), J. Cons. Int. Explor. Mer 28: 211–240.

Brielmann, N. 1981. Quantitative Untersuchungen an den Larven des Rügenschen Frühjahrsherings (Clupea harengus L.) im Greifswalder Bodden und angrenzenden Gewässern. Inaugural - Dissertation. Universität Rostock. 142 pp.

Brielmann, N. 1986. Über die Nutzbarkeit der quantitativen Larvenanalyse beim Rügenschen Frühjahrshering für die 0-Gruppeneinschätzung in den ICES-Untergebieten 22 + 24. Fischerei – Forschung 24(2): 20-21.

Brielmann, N. 1989. Quantitative analysis of Rügen spring-spawning herring larvae for estimating 0-group herring in Subdivisions 22 and 24. Rapp. P. v. Reun. Cons. Int. Explor. Mer, 190: 271-275.

Buckmann, K.; Pfeiffer, K. 1995: Hydrographische Messungen, Datenauswertung und –aufbereitung sowie Simulation der Hydrodynamik und der Wasseraustauschprozesse. Forschungsverbund Mecklenburg-Vorpommersche Küste, GOAP, Greifswalder Bodden und Oderästuar – Austauschprozesse, 2. Statusseminar, Greifswald, 13./14.12.1995, Kurzfassung des Vortrags.

Busch, A. 1993: Nahrungsökologische Untersuchungen an den Larven des Rügenschen Frühjahrsherings (Clupea harengus L.) im Greifswalder Bodden in den Jahren 1990 bis 1992. Diss., Universität Rostock, Mathematisch-Naturwissenschaftliche Fakultät, 132 S.

Fennel, W. 2001. Modeling of copepods with links to circulation models. Journal of plankton research, Vol. 23, No. 11:1217-12323.

Fennel, W., Neumann, T. 2003. Variability of copepods as seen in a coupled physical-biological model of the Baltic Sea. ICES Marine Science Symposia, 216:1-12.

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Fossum, P. 1996. A study of first-feeding herring (Clupea harengus L.) larvae during the period 1985 – 1993. ICES J. mar. Sci. 53:51-59.

Gröger, J. Schnack, D., Rohlf, N. 2001. Optimisation of survey design and calculation procedure for the international herring larvae survey in the North Sea. Arch. Fish. Mar. Res. 49 (2): 103-116p.

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Klinkhardt, M. 1984. Untersuchungen zur Embryonalphase des Laiches Rügenscher Frühjahrsheringe unter besonderer Berücksichtigung natürlicher Mortalitätsraten

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auf einem ausgewählten Laichplatz im Greifswalder Bodden. Dissertation Universität Rostock, 123 pp.

Klinkhardt, M. 1986. Gedanken zur Abhängigkeit der Laichentwicklung Rügenscher Frühjahrsheringe. Fisch.-Forsch. 24 (2): 22-27.

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Laine, P.; Eklund, J.; Lahdes, E.; Parmanne, R.; Rajasilta, M. 2003: Variation in ovarian fat in Baltic herring: is there a connection between herring fishery and success of reproduction ? Annales Universitatis Turkuensis AII 166, VI: 1-16.

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Tables Table 1: Total number of larvae (in billion) of different development stages of larvae and the quotients of the different development stages (N10 – hatched larvae, N15 – larvae which survived the transition to active feeding, N20 – new year-class index)

Year N10 [109]

N15 [109]

N20 [109]

N15 / N10 N15 / N20

1992 7.3 1.3 1.1 0.13 0.81 1993 25.2 17.7 2.3 0.73 0.13 1994 95.0 31.3 12.5 0.23 0.40 1995 80.7 22.9 8.0 0.18 0.35 1996 176.0 43.1 21.1 0.17 0.49 1997 56.1 13.5 4.5 0.25 0.33 1998 44.6 23.7 16.1 0.28 0.68 1999 54.6 36.1 19.1 1.09 0.53 2000 11.6 4.9 2.9 0.35 0.60 2001 37.3 14.5 4.8 0.29 0.33 2002 36.5 14.5 10.7 0.38 0.74 2003 27.1 8.2 5.0 0.45 0.61 2004 26.6 11.8 5.9 0.20 0.50 2005 41.6 14.4 3.9 0.28 0.27 2006 30.6 10.2 3.6 0.41 0.35 2007 55.4 5.2 1.9 0.05 0.37

Table 2: Maximum density of hatched larvae by station and maximum of mean hatched larvae by strata (S1: Strelasund, S2 – S5: strata of the Greifswalder Bodden, see Figure 1).

Year Maximum S1 S2 S3 S4 S5 1992 95 25.3 4.7 3.0 4.0 5.0 1993 95 24.9 21.6 45.7 16.9 11.7 1994 396 205.8 25.8 85.1 54.5 28.5 1995 452 247.4 23.6 42.9 29.7 56.4 1996 4663 1412.2 18.5 90.2 47.0 32.2 1997 217 96.6 16.7 21.2 30.0 44.8 1998 252 119.6 23.2 23.1 14.7 15.4 1999 259 46.8 86.8 47.1 18.0 23.5 2000 53 15.1 6.4 6.6 5.3 3.8 2001 71 40.2 13.8 25.1 22.1 10.7 2002 224 110.4 19.0 10.7 10.4 13.0 2003 75 31.2 19.6 6.5 20.9 16.0 2004 74 55.8 7.7 16.9 20.5 8.1 2005 355 169.2 27.7 30.6 10.3 9.7 2006 70 30.6 26.9 17.3 17.1 18.2 2007 2775 574.1 4.3 8.8 7.0 2.7

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Table 3. Proportion of index N20 by stratum and year. In addition the proportions of the northern and southern part of the Greifswalder Bodden summarized (S1: Strelasund, S2 – S5: strata of the Greifswalder Bodden, see Figure 1).

Year Stratum I S1

Stratum II S2

Stratum III S3

Stratum IV S4

Stratum V S5

S3+S4 S2+S5

1992 35 13 12 19 21 31 35 1993 33 13 26 22 7 48 19 1994 19 18 30 26 7 56 25 1995 30 5 29 31 5 60 10 1996 37 11 22 27 4 48 14 1997 30 11 28 24 7 52 18 1998 25 10 30 31 5 61 15 1999 34 12 27 21 5 49 17 2000 29 16 29 20 6 49 22 2001 19 19 22 27 13 50 32 2002 34 9 20 31 5 51 15 2003 38 8 11 38 6 48 13 2004 17 16 32 30 5 62 21 2005 38 10 25 22 5 47 15 2006 28 15 36 14 7 50 22 2007 39 10 26 18 7 44 16 Mean 30 12 25 25 7 50 19

Table 4: Proportion of hatched larvae and index of hatched larvae N10 by period and year

Year Week 14 to 17 Week 18 to 21 Week 22 to 26 N10 1992 25.6 74.4 0.0 14.3 1993 46.4 39.8 13.9 49.8 1994 10.6 76.8 12.6 187.6 1995 17.6 47.8 34.6 164.3 1996 0.9 78.1 21.0 350.2 1997 33.8 59.7 6.5 119.9 1998 0.8 78.4 20.9 88.7 1999 14.3 75.1 10.6 108.5 2000 18.7 61.5 19.7 23.2 2001 6.2 51.6 42.3 75.0 2002 18.6 71.9 9.4 77.6 2003 49.2 46.7 4.1 54.2 2004 16.6 77.7 5.7 52.5 2005 0.4 70.5 29.1 82.3 2006 20.6 57.0 22.5 62.9 2007 19.0 80.3 0.8 111.6

‐ 12 ‐ 

Figures

 

 Figure 1: Sampling locations. The Baltic Sea with ICES Sub-divisions and Strelasund and Greifswalder Bodden with the 35 fixed stations and the used new stratification (I .. V - the notation of the strata)

‐ 13 ‐ 

1994

1995

1996

2002

Figure 2: Density of N10 (hatched larvae) by cruises in 1994 to 1996 and 2000 by strata (DG – mean daily growth of larvae, S1: Strelasund, S2 – S5: strata of the Greifswalder Bodden, see Figure 1)

0

10

20

30

40

50

60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

16 17 18 19 20 21 22 23 24

Larv

ae p

er m

²

Mea

n da

ily g

row

th [m

m p

er d

ay]

Week

N15 - survívors by strata

DG S1 S2 S3 S4 S5

1994

0

10

20

30

40

50

60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

14 15 17 18 19 20 21 22 23 24La

rvae

per

m²

Mea

n da

ily g

row

th [m

m p

er d

ay]

Week

N15 - survívors by strata

DG S1 S2 S3 S4 S5

1995

0

20

40

60

80

100

120

0.00.10.20.30.40.50.60.70.8

19 20 21 22 23 24

Larv

ae p

er m

²

Mea

n da

ily g

row

th [m

m p

er d

ay]

Week

N15 - survívors by strata

DG S1 S2 S3 S4 S5

1996

051015202530354045

0.00.10.20.30.40.50.60.70.8

17 18 19 20 21 22 23 24 25

Larv

ae p

er m

²

Mea

n da

ily g

row

th [m

m p

er d

ay]

Week

N15 - survívors by strata

DG S1 S2 S3 S4 S5

2002

Figure 3: Density of N15 (larvae after successful transition to active feeding) by cruises in 1994 to 1996 and 2000 by strata (DG – mean daily growth of larvae, S1: Strelasund, S2 – S5: strata of the Greifswalder Bodden, see Figure 1)

‐ 14 ‐ 

N10 in billion

N15

in b

illio

n

0 3 6 9 12 15 18(X 10000)

0

1

2

3

4

5(X 10000)

N15 = 5372 + 0.232 * N10, R² = 63 % Total area

N10 (GB) in billion

N15

(GB)

in b

illio

n

0 2 4 6 8(X 10000)

0

0.5

1

1.5

2

2.5

3(X 10000)

N15 = 1304 + 0.375 * N10, R² = 73 % Greifswalder Bodden

N10_S1 in billion

N15

_S1

in b

illio

n

0 2 4 6 8 10 12(X 10000)

0

0.4

0.8

1.2

1.6

2(X 10000)

N15 = 1924 + 0.136 * N10, R² = 66 % Strelasund

Figure 4: Linear regressions between the indices N10 and N15 for the total area and the Greifswalder Bodden (GB) and the Strelasund (S1) (areas see Figure 1) based on 16 years

N15 in billion

N20

in b

illio

n

0 1 2 3 4 5(X 10000)

0

4

8

12

16

20

24(X 1000)

N20 = -649 + 0.49* N15, R² = 80 % Total area

N15 (GB) in billion

N20

(GB)

in b

illio

n

0 0.5 1 1.5 2 2.5 3(X 10000)

0

3

6

9

12

15(X 1000)

N20 = -475 + 0.48 * N15, R² = 72 % Greifswalder Bodden

N15 (S1) in billion

N20

(S1)

in b

illio

n

0 0.4 0.8 1.2 1.6 2(X 10000)

0

2

4

6

8(X 1000)

N20 = 249 + 0.43 * N15, R² = 83 % Strelasund

Figure 5: Linear regressions between the indices N15 and N20 for the total area and the Greifswalder Bodden (GB) and the Strelasund (S1) (areas see Figure 1) based on 16 years