Growth in common brown shrimp Crangon crangon (Caridea, L. 1758). Improving growth parameterization in future lifecycle models

Abstract

The common brown shrimp (Crangon crangon) is a species of high economical and ecological value in the southern North Sea ecosystem. Its presence in almost all areas, not only makes C. crangon a key species by being both predator and prey for a wide variety of other ecosystem inhabitants, but also forms the basis of a 500-vessel strong fishery that lands up to 40,000 tons annually, generating revenues of around 120 million euros. Due to the lack of moult enduring hard structures, a year-round lasting spawning period and the short-lived nature of the species, age determination in the common brown shrimp is nearly impossible. Hence, common-, age-based management approaches, such as the maximum sustainable yield (MSY) were found not applicable for the common brown shrimp stock. Aiming at the certification of the fishery by the marine stewardship council (MSC), in 2016 a self-management plan was introduced, in which measures alternative to a quota were set. The potential benefits and effects of these measures were calculated using a species-specific yield per recruit model, parameterized with growth data from decades of growth research on the species. The results of this model and the conclusions drawn for the management are highly influenced by the given growth input. Although the species, due to its extensive use, has been the subject of growth research since the late 1940th, to date questions remain about the factors influencing the growth in common brown shrimp. The aim of this thesis is to address some of these questions, and to further improve the parameterization in future growth calculations. Crucial for the survival and growth of common brown shrimp is known to be food availability. At the same time, parts of the common brown shrimp population are-, depending on season, known to be exposed to prolonged periods of starvation. Despite their omnivorous feeding behaviour and the otherwise very productive ecosystem, in winter up to 80 percent of the population can be in a starving condition. The response of C. crangon to starvation has studied previously, but no study has been conducted in which the animals were starved for long periods and then fed again. Manuscript 1 addresses the influence of prolonged food deprivation followed by re-feeding on the growth of the common brown shrimp. In laboratory experiments, animals were deprived of food for different periods of time and were re-fed thereafter. Subsequently, the moulting interval and the growth increment, determined individually, were compared with a control group that was fed ad libitum for the entire trial period. Food deprivation significantly increased the time between two successive moults and reduced the growth increment into a negative range. Shrinkage had previously been observed in the common brown shrimp occasionally but was first observed systematically in manuscript 1. In addition to the prolongation of the moulting interval during food deprivation, which was previously considered to mainly be a function of temperature and size, shrinkage was identified as a physiological necessity to compensate for the dry mass lost during the starvation period. The results from manuscript 1, combined with the observations of other authors that especially in winter up to 80 % of the population show a condition that indicate prolonged starvation, could be used to adjust growth predictions of the stock in winter. Since egg deposition and egg laying are linked to a moult event, the delayed moult due to starvation could affect the reproductive cycle of starving females. Reduced growth during starvation, and especially shrinkage could reduce catch quantities in spring fisheries, after winter with food scarcity. In addition to gaining a better understanding of the influence of starvation and re-feeding on growth, in Manuscript 2 the effects of in field density, a factor that could potentially trigger starvation in the shrimp stock, was investigated. Conserving the stock due to management measures, especially when reducing effort or increasing mesh size, leads to a local increase of individuals and hence density. Concerns towards density dependent growth counteracting potential benefits of a newly implemented measures, showed the need of further investigations into shrimp growth at different densities. In Manuscript 2, three different approaches, in which different proxies for growth were used, to compare growth potential in common brown shrimp caught at different densities. Besides laboratory growth observations and dry weight conditions from field samples the relative abundance of two successive length classes in samples from the demersal young fish survey (DYFS) were used to detect a potential decrease in growth, with increasing density. Growth proxies at different densities were compared within a short period of time (30-50 days), within month of one summer and between the same months in different years. In none of the three approaches indications for a potential growth limitation with increasing density was found. In summer, when the expected effects of a conserving measure are greatest, no signs for poorer growth at higher densities was found. On the contrary, animals sampled at the highest observed density usually showed the best growth potential. This led to the conclusion that some of the main factors determining population size are bottom up driven. Hence, do good conditions lead to high densities, rather than high densities tend to worsen the shrimps’ conditions. The fact that no deterioration in growth performance was observed at higher densities in any of the approaches, as well as the fact that the density in the field fluctuates far more naturally, than it would with a sparing measure, led to the conclusion that concerns about loss of profit with density limitation are unjustified. While density did not serve as an explanation for any of the observed growth differences between different growth trials, the seasonal origin of the sampled individuals was found valid to do so. During the analysis of the laboratory experiments in the context of density limitation of growth, there was a striking pattern towards particularly well growing animals in different length classes, recognisable through the season. In 2011, Hufnagl and Temming published laboratory experiments that pointed to a so-called cohort effect. Animals of the same length grew very differently, which was attributed to the fact that they were caught five weeks apart, and belonged to two different cohorts. Since different growth rates within the season would have an impact on the calculations to determine stock development, the focus of Manuscript 3 was on the investigation of the so called “Cohort Effect”. Growth potential, both as a direct measure from laboratory experiments and derived from a set of dry weight condition data was compared for individuals of the common brown shrimp (Size 20-70 mm), and during the season of different years between 2006 and 2021. Both the growth experiments as well as the analysis of dry weight data, pointed at a specific cohort being consistently responsible for the highest growth observations throughout the season. Larger increments as well as shorter moulting intervals at the same experimental conditions, were found in individuals that were supposedly hatched from winter eggs. The cohort effect was observed to be stronger in some years of the time series than in others. Especially in 2019, which was also an exceptional year from the fishery point of view, no growth differences were observed within the season. The enormous shrimp stock in 2018, as well as a relatively cold winter in 2018/19, were identified as potential reasons for the absence of the cohort effect in 2019. Implications for growth calculations were highlighted, and the protection of specifically the well-growing cohort was identified as a possible management objective. In Manuscript 4 the development of a method to determine moult interval in common brown shrimp, from frozen field samples, was intended to allow in situ growth rate without the necessity of costly and elaborate growth experiments. Besides growth increment, the actual increase in length, moulting interval determines the frequency at which the length increase occurs. Hence knowing the period between two successive moults, is essential for growth rate calculations. Since the determination of moulting interval requires time consuming and elaborate laboratory experiments, and the results are error prone to the housing condition, the need for a method of in situ moulting interval determination from field samples is high. This is also the reason, why a large part of the observed moulting intervals was not measured individually but calculated on the basis of animals moulting within a short period of time. Based on this principle, the moult interval for a given length class should be calculated using recently moulted animals within a field sample. To identify the freshly skinned animals, a device was developed that could empirically determine the carapace hardness using a method for hardness determination known from materials science. By validating the device with moulting observed in the laboratory, a method was developed based on which it is possible to identify a recent skinning by means of carapace hardness and dry weight.