Age Validation of the European Plaice (Pleuronectes platessa L
1
Age Validation of the European Plaice (Pleuronectes platessa L.) from the Celtic Sea and the North Sea: Using Stable δ 18 O isotope in otoliths Sheku Sei 1* , Pierre Degeest 1 , Wim Demare 2 and Philippe Claeys 1 1 Ecological Marine management, Faculty of Science, Vrije Universiteit Brussel, Pleinlaan 2, 1050, Brussels Belgium 2: Sea Fisheries Department, Oostende, Belgium ABSTRACT RESULTS METHODS 1 METHODS 2 Accurate age estimation techniques are of crucial importance in fisheries research if the data collected is to support management policies. Apart from classical aging by interpretation of growth zones in fish otoliths, age validation studies can be enhanced by the use of stable isotopes in otoliths. This is useful when studying commercial fish species that are difficult to age with classical methods. The stable isotope approach presented here validates conventional aging of two plaice stocks (Pleuronectes platessa L.) from the Celtic Sea and the North Sea. Plaice saggital otolith samples (989 in total) selected for this study were obtained from the archived collection at the Sea Fisheries Department, Ooshtend, Belgium. Otoliths were sectioned via a labcut machine and ages obtained through image analysis system (AxioVision 3.1) by counting successive opaque and translucent growth bands in the otolith section. A total of 12 otoliths from the band count aging were selected for stable isotope analysis. These samples were analyzed at the stable isotope laboratory at the Department of Geology, Vrije Universiteit Brussel (VUB), Belgium. The Merchantek Micromill device permitted milling of time specific carbonate samples from otolith growth bands (1-3 samples per growth layer). The Kiel carbonate device coupled to the stable isotope mass spectrometer, allowed final analysis of oxygen isotope signals from the carbonate sample aliquots. Seasonal variations in the δ 18 O signals were used to age the otoliths and to validate timing of ring formation. This δ 18 O –aging procedure was based on counting the annual δ 18 O minima in successive growth bands of the otoliths. In most cases, the δ 18 O-aging coincided well with the band count ages except for one otolith from the North Sea, for which band counting produced an age of 8 years while the δ 18 O technique indicated 7 years. This underlines the difficulty in the determination of ages of older fishes. For the validation of the timing of ring formation, regional salinities and otolith δ 18 O signals were used in a paleotemperature model. The otolith-derived temperatures were very close to the temperatures in the ambient seawater inhabited by plaice, and these temperatures have validated the timing of ring formation in the otoliths, Characterizing the opaque zone with higher temperatures and the translucent zone with lower temperatures. This shows that the opaque zone was laid down during summer while the translucent zone was laid down during winter. Plaice otolith is therefore a faithful recorder of ambient seawater temperature and can be a valuable tool for fisheries management. INTRODUCTION AND OBJECTIVES Sectioned otoliths from the archived collection at the Sea Fisheries Department, Ostende, Belgium, were mounted and affixed on to glass slides and viewed from above on black background under reflected light using a stereoscopic microscope, coupled to a computer that was powered with an image analysis software AxioVision 3.1. This allowed a real time visualization of growth features (Fig.1 A). To examine the timing of ring formation, and the potential of otoliths as recorders of ambient temperatures, temperature records were derived from δ 18 O values of plaice otoliths using the paleotemperature model published by Patterson (1993) for otolith aragonite: 10 3 ln(α) =18.56 {(10 3 T -1 K)} –33.49 The model entails empirical fractionation relationships ( Grossman and Ku, 1986; Patterson et al., 1993; Thorrold et al., 1997), assuming that the δ 18 O of the water has remained unchanged throughout the period of investigation - an unambiguous interpretation of changes in the δ 18 O of the aragonite or calcite: α is the fractionation factor between water and aragonite and T is temperature in Kelvin. The value of α is given as: α = (δCaCO 3 + 10 3 )/ (δH 2 O + 10 3 ) δCaCO 3 is δ 18 O VPDB of otolith aragonite and δH 2 O is δ 18 O sw(VSMOW). The conversion of δ 18 O values from the VPDB scale to the VSMOW scale was obtained using the model proposed by by Friedman and O’Neil (1977): δ 18 O (VSMOW) = 1.03091 ( δ 18 O VPDB) + 30.91 Fishing removes various age classes from an exploited fish population, distorting its age structure. This can affect the reproductive capacity of the population because age is crucial for fecundity and mortality of fishes (Pitcher and Hart, 1982). The foundation of virtual population analysis (VPA) model heavily relies on catch-at-age time series. Any error in age determination can cause biases in the output of VPA. The International Council for the Exploration of the Sea (ICES) gives scientific advice for the management of more than 100 commercial fish and shellfish stocks in the North-East Atlantic (NEA). For most stocks, the advice is relying on age-based assessments using otoliths. Otoliths are fish ear stones comprising of three pairs of white carbonate bodies (lapilli, asterisci and saggitae) found in the head of all bony fishes. Saggital otoliths are the largest and they display rhythmic growth patterns. As the fish grows, new rings of calcium carbonate are laid down, one layer per year. In the otoliths of North Atlantic fish species including plaice, the opaque zone has generally been referred to as ‘summer zone’ or ‘fast growth zone’ because of their larger width, which are mostly laid down during periods of fastest growth (summer). The translucent zone (hyaline zone) has been considered as the ‘winter zone’ or ‘slow growth zone’; because they have smaller width and they are laid down during periods of slow growth (winter) (Jensen, 1965) . Apart from classical aging by interpretation of growth zones in fish otoliths, age validation studies can be enhanced by the use of stable isotopes stored in otoliths The oxygen isotope ratios in the otolith of fishes vary with temperature and are deposited in equilibrium with the ambient seawater. They can be used to validate annual rings in fishes.(Thorrold et al, 1997 Patterson et al, 1999; Weidmann and Millner, 2000). The usefulness of stable isotope approach is that aging is difficult in some marine fishes because annuli cannot be easily discerned. For fish stocks such as the European plaice that has been classified as suffering from reduced reproductive capacity and at risk of being harvested unsustainably, there can be a decrease in size-at-age, as a response to fishing pressure (Law and Rowell 1993, Rochet 1998, Sinclair et al., 1999, Olsen et al., 2004). (ICES, 2005). The major objective of this study is to determine ages of two plaice stocks in the Celtic Sea and the North Sea, using the classical approach of layer counting, and to validate the timing of ring formation in plaice otoliths using Paleotemperature models and δ 18 O isotopic signatures in otoliths. A total of 12 otoliths were used for stable isotope analysis. The Merchantek MicroMill device equipped with a drill bit of 300µm was used to mill time specific aragonite samples from the otoliths. The glass slide with sectioned otoliths was attached to a firmly screwed micro milling stage by the help of adhesive tapes, and viewed on a computer monitor via a colour video camera (Fig.1B), mounted on the microscope of the system. The optical medium is reflected light. The opaque zone is bright and translucent zone is dark. The drill bit diameter is 300µm A B Fig. 1 δ 18 O aging chronologies The stable oxygen isotope time series in the otolith annuli displayed a clear pattern of seasonality in all otoliths studied. Otolith 020446D22: Celtic Sea plaice (7yr-old) Otolith 1104269D64: North Sea plaice (7yr-old) •All ages determined except one coincided with the band count aging •The opaque (wider) bands were generally characterized with lower δ 18 O and higher temperature signals compared to the translucent (narrow) bands. The opaque layers for plaice are laid down during warmest periods (fast growth periods) where as the translucent bands are precipitated mostly during the colder periods of the year
Transcript of Age Validation of the European Plaice (Pleuronectes platessa L
Age Validation of the European Plaice (Pleuronectes platessa L.) from the Celtic Sea and the North Sea: Using Stable δ 18O isotope in otoliths
Sheku Sei1*, Pierre Degeest1, Wim Demare2 and Philippe Claeys1
1Ecological Marine management, Faculty of Science, Vrije Universiteit Brussel, Pleinlaan 2, 1050, Brussels Belgium2: Sea Fisheries Department, Oostende, Belgium
ABSTRACT RESULTSMETHODS 1 METHODS 2
Accurate age estimation techniques are of crucial importance in fisheries research if the data collected is to support management policies. Apart from classical aging by interpretation of growth zones in fish otoliths, age validation studies can be enhanced by the use of stable isotopes in otoliths. This is useful when studying commercial fish species that are difficult to age with classical methods. The stable isotope approach presented here validates conventional aging of two plaice stocks (Pleuronectesplatessa L.) from the Celtic Sea and the North Sea. Plaice saggital otolith samples (989 in total) selected for this study were obtained from the archived collection at the Sea Fisheries Department, Ooshtend, Belgium. Otoliths were sectioned via a labcutmachine and ages obtained through image analysis system (AxioVision 3.1) by counting successive opaque and translucent growth bands in the otolith section. A total of 12 otoliths from the band count aging were selected for stable isotope analysis. These samples were analyzed at the stable isotope laboratory at the Department of Geology, Vrije Universiteit Brussel (VUB), Belgium. The Merchantek Micromill device permitted milling of time specific carbonate samples from otolith growth bands (1-3 samples per growth layer). The Kiel carbonate device coupled to the stable isotope mass spectrometer, allowed final analysis of oxygen isotope signals from the carbonate sample aliquots. Seasonal variations in the δ18O signals were used to age the otoliths and to validate timing of ring formation. This δ18O –aging procedure was based on counting the annual δ18O minima in successive growth bands of the otoliths. In most cases, the δ18O-aging coincided well with the band count ages except for one otolith from the North Sea, for which band counting produced an age of 8 years while the δ18O technique indicated 7 years. This underlines the difficulty in the determination of ages of older fishes. For the validation of the timing of ring formation, regional salinities and otolith δ18O signals were used in a paleotemperature model. The otolith-derived temperatures were very close to the temperatures in the ambient seawater inhabited by plaice, and these temperatures have validated the timing of ring formation in the otoliths, Characterizing the opaque zone with higher temperatures and the translucent zone with lower temperatures. This shows that the opaque zone was laid down during summer while the translucent zone was laid down during winter. Plaice otolith is therefore a faithful recorder of ambient seawater temperature and can be a valuable tool for fisheries management.
INTRODUCTION AND OBJECTIVES
Sectioned otoliths from the archived collection at the Sea Fisheries Department, Ostende, Belgium, were mounted and affixed on to glass slides and viewed from above on black background under reflected light using a stereoscopic microscope, coupled to a computer that was powered with an image analysis software AxioVision3.1. This allowed a real time visualization of growth features (Fig.1 A).
To examine the timing of ring formation, and the potential of otoliths as recorders of ambient temperatures, temperature records were derived from δ18O values of plaice otoliths using the paleotemperature model published by Patterson (1993) for otolith aragonite:
103ln(α) =18.56 {(103T-1K)} –33.49
The model entails empirical fractionation relationships ( Grossman and Ku, 1986; Patterson et al., 1993; Thorrold et al., 1997), assuming that the δ18O of the water has remained unchanged throughout the period of investigation - an unambiguous interpretation of changes in the δ18O of the aragonite or calcite:
α is the fractionation factor between water and aragonite and T is temperature in Kelvin. The value of α is given as:
α = (δCaCO3 + 103)/ (δH2O + 103)
δCaCO3 is δ18OVPDB of otolith aragonite and δH2O is δ18Osw(VSMOW).
The conversion of δ18O values from the VPDB scale to the VSMOW scale was obtained using the model proposed by by Friedman and O’Neil (1977):
δ18O(VSMOW) = 1.03091 ( δ18OVPDB) + 30.91
Fishing removes various age classes from an exploited fish population, distorting its age structure. This can affect the reproductive capacity of the population because age is crucial for fecundity and mortality of fishes (Pitcher and Hart, 1982). The foundation of virtual population analysis (VPA) model heavily relies on catch-at-age time series. Any error in age determination can cause biases in the output of VPA. The International Council for the Exploration of the Sea (ICES) gives scientific advice for the management of more than 100 commercial fish and shellfish stocks in the North-East Atlantic (NEA). For most stocks, the advice is relying on age-based assessments using otoliths. Otoliths are fish ear stones comprising of three pairs of white carbonate bodies (lapilli, asterisci and saggitae) found in the head of all bony fishes. Saggital otoliths are the largest and they display rhythmic growth patterns. As the fish grows, new rings of calcium carbonate are laid down, one layer per year. In the otoliths of North Atlantic fish species including plaice, the opaque zone has generally been referred to as ‘summer zone’ or ‘fast growth zone’ because of their larger width, which are mostly laid down during periods of fastest growth (summer). The translucent zone (hyaline zone) has been considered as the ‘winter zone’ or ‘slow growth zone’; because they have smaller width and they are laid down during periods of slow growth (winter) (Jensen, 1965) . Apart from classical aging by interpretation of growth zones in fish otoliths, age validation studies can be enhanced by the use of stable isotopes stored in otoliths The oxygen isotope ratios in the otolith of fishes vary with temperature and are deposited in equilibrium with the ambient seawater. They can be used to validate annual rings in fishes.(Thorrold et al, 1997 Patterson et al, 1999; Weidmann and Millner, 2000).The usefulness of stable isotope approach is that aging is difficult in some marine fishes because annuli cannot be easily discerned.
For fish stocks such as the European plaice that has been classified as suffering from reduced reproductive capacity and at risk of being harvested unsustainably, there can be a decrease in size-at-age, as a response to fishing pressure (Law and Rowell 1993, Rochet1998, Sinclair et al., 1999, Olsen et al., 2004). (ICES, 2005). The major objective of this study is to determine ages of two plaice stocks in the Celtic Sea and the North Sea, using the classical approach of layer counting, and to validate the timing of ring formation in plaice otoliths using Paleotemperature models and δ18O isotopic signatures in otoliths.
A total of 12 otoliths were used for stable isotope analysis. The Merchantek MicroMilldevice equipped with a drill bit of 300µm was used to mill time specific aragonite samples from the otoliths. The glass slide with sectioned otoliths was attached to a firmly screwed micro milling stage by the help of adhesive tapes, and viewed on a computer monitor via a colour video camera (Fig.1B), mounted on the microscope of the system. The optical medium is reflected light. The opaque zone is bright and translucent zone is dark. The drill bit diameter is 300µm
A
BFig. 1
δ18O aging chronologies
The stable oxygen isotope time series in the otolith annuli displayed a clear pattern of seasonality in all otoliths studied.
Otolith 020446D22: Celtic Sea plaice (7yr-old)
Otolith 1104269D64: North Sea plaice (7yr-old)
•All ages determined except one coincided with the band count aging
•The opaque (wider) bands were generally characterized with lower δ18O and higher temperature signals compared to the translucent (narrow) bands. The opaque layers for plaice are laid down during warmest periods (fast growth periods) where as the translucent bands are precipitated mostly during the colder periods of the year
