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Hufnagl M.,Institute for Hydrobiology and Fishery Science | Temming A.,Institute for Hydrobiology and Fishery Science | Siegel V.,Johann Heinrich Von Thunen Institute | Tulp I.,Wageningen IMAResearch Institute for Marine Resources and Ecosystem Studies | Bolle L.,Wageningen IMAResearch Institute for Marine Resources and Ecosystem Studies
ICES Journal of Marine Science | Year: 2010

Total mortality (Z, year-1) of southern North Sea brown shrimp (Crangon crangon) was determined as Z = θK, based on the von Bertalanffy length-growth constant (K, year-1) and θ derived from length-based methods. Mortality estimates were based on length frequency distributions obtained from four long-term dataseries (1955-2006): German Demersal Young Fish Survey, Dutch Demersal Fish Survey, and two German Bycatch series (Büsum and East Frisia). Four methods to estimate θ and L∞ were evaluated. Highest total mortality (Z = 8 year-1) was estimated for the early 1990s, and the lowest (Z = 4 year-1) for the 1960s. Accounting for these differences, a median Z rather than mean values was calculated for the whole series, and the value ranged from 5.74 (Ssentongo and Larkin method), through 5.65 (Beverton and Holt method) and 5.64 (Jones and Zalinge method), to 5.35 (length-converted catch curves). Over the whole period, an increase in θ and a decrease in the proportion of shrimps >60 mm in the catch was observed, whereas asymptotic length L∞ remained constant (at 79.3 mm total length). © 2009 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. Source


Hufnagl M.,Institute for Hydrobiology and Fishery Science | Temming A.,Institute for Hydrobiology and Fishery Science | Danhardt A.,Institute of Avian Research
Marine Biology | Year: 2010

This study was undertaken to further clarify whether the brown shrimp, Crangon crangon (Linnaeus 1758), is a gonochorist, a facultative or an obligate hermaphrodite. Juvenile shrimps were sampled from intertidal habitats along the German Wadden Sea coast with a push net and from a power plant water inlet to quantify the share of primary females. Length-based sex ratios were determined for about 27,000 individuals using external characteristics. Observed sex ratios were mainly female-biased, and also large males occurred regularly in the catch. This indicates that sex at hatch is not male as would be characteristic for an obligate protandric hermaphrodite and that not all male shrimps change sex. A cohort-based computer simulation, including sex-specific growth rates, mortality and seasonally varying recruitment, generated sex ratios comparable to the field. The observed decline in the proportion of males with increasing size can be explained solely by faster growth of females without involving hermaphroditism. Based on temperature-dependent growth and moult rates as well as length-specific numbers of eggs per female, the potential egg production of primary and secondary females was modelled, yielding contributions of secondary females of <1%. Sex change in C. crangon has previously been observed and may be interpreted as an evolutionary relict of this species having evolved in a habitat characterized by lower population densities, lower predation levels and increased longevity compared to today's living conditions in North Sea coastal waters. © 2010 Springer-Verlag. Source


Hufnag M.,Institute for Hydrobiology and Fishery Science | Temming A.,Institute for Hydrobiology and Fishery Science
Marine Ecology Progress Series | Year: 2011

Laboratory experiments were performed to determine the growth rates of Crangon crangon as a function of total length (L = 20 to 60 mm) and temperature (T = 5, 10, 15, 20 and 25°C) under ad libitum feeding conditions. Mean (±SD) growth rates ranged from 0.04 ± 0.03 to 0.56 ± 0.1 mm d-1 at 5 and 25°C, respectively. Unexpectedly, the catch date also influenced growth rates, indicating that recently recruited animals grew faster than overwintering shrimps of the same size. Female shrimps of the recent recruitment wave showed significantly higher growth rates than male shrimps. Individual moult intervals were determined using a marking method. Mean intervals (±SD) varied between 8 ± 3.6 and 45 ± 7.6 d for 30 mm shrimps at 25 and 5°C, respectively. Temperature and length affected the moult interval but not the moult increment. Variability in the observed growth rates at a given length and temperature was mainly an effect of variable moult increments. Results from 2 pre-experiments also indicate an effect of food quality on growth, with shrimps growing faster when feeding on live copepods in comparison to several other food sources. © Inter-Research 2011. Source


Hufnag M.,Institute for Hydrobiology and Fishery Science | Temming A.,Institute for Hydrobiology and Fishery Science
Marine Ecology Progress Series | Year: 2011

Existing laboratory and field data on growth were combined, reanalyzed and discussed to generate a holistic temperature-, length- and gender-dependent growth rate (G, mm d -1) model for North Sea region brown shrimp Crangon crangon (L.). Length (L, mm) and temperature (T, °C) dependent growth rates of Crangon crangon are highly variable within and among studies but decrease with L and increase with T. Applying general nonlinear regression, mean growth was derived as G = 0.02421·T - 0.00115·e 0.08492·T·L (r 2 = 0.860). Applying quantile regression (75th percentile), a growth model describing growth of the fastest growing fraction of the population was derived as Gmax = 0.03054·T - 0.00104·e0.09984·T·L (r 2 = 0.857). Female growth rates were higher than male growth rates and were similar to Gmax. In a simulation, G and Gmax were used with seasonally varying temperature to generate monthly length trajectories (cohorts). Further, length-based mortality was included and the fraction of each cohort attaining minimal commercial size was calculated. May cohorts (5 mm initial length), representing spring recruitment, grew to 50 mm by November if G was used. Application of the fast growth model (Gmax) allowed for the same length to be reached 2 mo earlier. We conclude that the autumnal peak in adult abundance in the North Sea is most probably due to recruitment from the spring cohort of the same year. Our results suggest that the previous year's summer cohort contributes little to this autumnal peak because of high cumulative and overwintering mortality. © Inter-Research 2011. Source


Temming A.,Institute for Hydrobiology and Fishery Science | Hufnagl M.,Institute for Hydrobiology and Fishery Science
ICES Journal of Marine Science | Year: 2015

The North Sea brown shrimp fishery is currently regulated neither with quotas nor with effort management. The current paradigm of non-management was based on an analysis of the total predation by cod and whiting in relation to commercial catches for the period 1970-1995 and the estimated total dominance of natural mortality. However, since this period, the North Sea ecosystem has undergone pronounced changes with overfishing and climate change causing a substantial decline in predator stocks, namely cod and whiting. In addition, both predators have shifted their range of distribution causing a reduced overlap with brown shrimp. Here, we extend the previous assessment of brown shrimp predation for the years 1996-2011 using updated stock assessment and predator distribution data. For the first time, predation estimates are used together with commercial landings to partition independent estimates of total mortality into fishing and predation mortality. We demonstrate that the decline of key predators of brown shrimp in combination with a shift in the distributional range of the predators has caused a new situation, in which the fishery has become the main mortality source of adult brown shrimp (>50 mm). Average landings since 2000 have been ∼40% higher than in the 1980s and 1990s, indicating that humans have at least partly taken over the share previously taken by juvenile whiting and cod. We discuss that this situation is likely to continue, because three marine mammal species have built up a combined population of over 80 000 individuals, which hunt for potential brown shrimp predators mainly in the distribution area of brown shrimp. The application of two yield-per-recruit models of different complexity indicates potential growth overfishing of brown shrimp and reopens the discussion of management. © International Council for the Exploration of the Sea 2014. All rights reserved. Source

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