Data on the population dynamics of plaice (Pleuronectes platessa) off the coast of east Anglesea and North Wales between Point Lynas and Conway Bay is presented (Fig. 1). The study is based on analyses from a total number of 5,125 fish taken from October to December each year from 2003 to 2009. These findings show that plaice was significantly more abundant inshore than offshore. Size-age frequencies show females on average are older and longer than the males. Females were generally larger and heavier than males but length -weight relationships and growth curves are similar for both sexes. Mortality rate was significantly higher for males than females. Sex ratios show more females than males and age/size at maturity show females mature younger and they are longer in size than males. The analyses is relevant both for fish population modelling and management strategies for sustainable harvesting of this commercially important dermesal stock.
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Although there is some mixing between biological stocks of plaice, (Dunn and Paulson, 2002) these dermesal fish form fairly distinct populations inhabiting specific geographical areas with particular spawning areas.The plaice stock sampled in this study forms part of the Irish Sea stock (ICES division VIIa). The ICES precautionary approach to fisheries management, based on assessments of spawning stock biomass (SSB) puts this area within safe biological limits,and is considered to be harvested sustainably and at full reproductive capacity (ICES, 2009). Although mortality for this species has declined through the implementation of fish management strategies with landings dropping from 3,300 tonnes in 1992 to 600 tonnes in 2008 and with a corresponding rise in spawning stock biomass (ICES, 2009) this stock has been and still is heavily fished. Understanding of population dynamics is important in assessing the long term status of plaice stocks,maintaining sustainable commercial yields and reducing any biological risks within the population.
Collection of Samples
Samples were taken for this analyses each year from 2003-2009 from five areas along the coast of North Wales and East Anglesey. (Fig 1 ) These comprised three inshore areas; Red Wharf Bay, Conwy Bay and inshore Colwyn Bay and two offshore areas; offshore Colwyn Bay and Point Lynas.Using a mesh size 7.6 cm, a rock hopper otter trawl was towed for periods of 60 minutes at average speeds of 2-3 (up to 5) knots. The plaice were measured to the nearest mm and a length stratified sub-sample was taken using three fish for each 1cm length class which was later weighed in the laboratory to the nearest gram and dissected to determine sex and stage of maturity. Ages of the fish were determined through extraction of the otoliths, immersing them in histoclear and identifying and counting growth bands under a binocular microscope.
In order to use a significantly large sample size for the data set, the data has been combined into one large data set (total 5,125 fish) which was then divided into males and females. Abundances for each of the five areas were calculated in terms of the number of plaice caught per hour. Comparison of average lengths between years was made using a one-way ANOVA with a posthoc Tukey's test. Comparison of lengths between males and females was made using sex ratios for each length class from the sub-samples. The lengths of offshore and inshore fish were compared. Yate's corrected Chi-squared tests were used to compare size/age frequencies for both sexes. This statistical test was also used to compare the lengths of fsh caught in the inshore and off-shore areas and a Mann-Whitney U test was used to compare the average ages of these fish.
The equation W= aLb where W=weight, L=length and a and b are constants was applied to determine the length//weight relationship for males and females. A linear regression was performed on log transformed data for males and females. These slopes were compared using a general linear model (GLM) where sex was the co-variant. A t-test was used to see if values from the general linear model were significantly different from isometric growth (a b value of 3 signifies isometric growth)
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A Von Bertalanffy equation Lt = Lmax (1-e-K(t-t0)) was used to construct growth curves for each sex. Lt is the average length of fish at a given age (t), Lmax is the asymptotic total length, K the growth coefficient and t0 the age at length 0. A likelihood ratio test (Kimura, 1980) was applied to compare the growth parameters of males and females. Where age classes had only 1 or 2 individuals they were not included.(fish over 7.5).
Male and female total instantaneous mortality rates (Z, year-1) were calculated using linearised catch curves. A GLM was used to look for differences between the sexes. Due to the nature of the fishing gear being used, smaller fishes aged 0.5 and 1.5 years were only caught later in the trawl when the net had become clogged with larger fish and were were excluded as unrepresentative as were age classes with only 1 or 2 individuals.
Sex ratio and age/size at 50% maturity was calculated using the logistic equation P = 1/(1 + exp[-r(X-X50)]), where P is the proportion of mature individuals in a given length or age class, X50 the age or length at 50% maturity and r is a constant. This was calculated by separating the fish into length classes each spanning 3cm. Spurious results were probably human error in ageing or recording and were ignored.
The results show 110 plaice were caught per hour (inshore) compared to 39 plaice caught per hour (off-shore) (t=3.26, 29 d.f., p=0.003) demonstrating a greater abundance of plaice in the inshore water areas. (Table 1)
Size / age frequencies show that offshore fish were significantly older than those found in the inshore waters (W = 1457590.500, p<0.01). No fish in their first year of life (0-group fish) were found in offshore waters. Smaller fish(16cm or less) were predominately caught in the inshore areas but these areas also were populated by the larger length ranges (32cm-41cms). The size range 17cm-26cm was found predominately inshore but also in fewer numbers offshore.
Size-age frequencies (Fig. 2 and Tables 2 and 3 ) show females are more numerous and on average are older and longer than the males.In the younger classes (0.5-1.5years) males predominate. In the 2.5 year age range the numbers of males and females are equal. However, females were predominant among fish aged 3.5 years or older. The total number of females aged was 1136 whereas the total number of males aged was only 748. The oldest fish caught were 7.5 years. No males were caught older than 6.5 years. (Table 3)
Male and female length-weight relationships (Fig. 3) produced significantly different linearised slopes (F= 24.851, p <0.01) showing a significant interaction between sex and length (F= 74.944, p<0.01). Isometric growth was exibited in both males and females sexes exhibited isometric growth,males: t=-0.014, 752 d.f., p=0.989, females: t=1.05, 1130 d.f., p=0.294).
There is a significant difference (χ² = 26.417, 3 d.f., p < 0.01)in the Von Bertalanffy growth curves for males and females (Fig 4 ). This effect was produced by the fact that females have reached a significantly larger asymptotic total length compared to males (χ² = 50.509, 1 d.f., p < 0.01). There was no significant difference between K and t0 in males and females.
The instantaneous mortality (Z/year) rates obtained from the linearised catch curve were significantly higher (1.27) for males (F= 19.283, p=0.014) than for females (0.762) (Fig. 5).
The age / size at maturity (Figs. 6 and 7) showed that both sexes matured at a similar age with 50% maturity at 3.21 years in females and 3.57 years in males.However, males appeared to mature at a slightly smaller size with 50% mature at 29.5cm compared to 32cm in females.
There appears to be a slight reduction in mean total length over time from 2003-2009 size (F = 44.935, p<0.01) (Fig. 8). Fish caught in 2003 were longer than those caught in 2009 with respective average lengths of 26cm compared to an average of 23cm in 2009.
The greatest abundance of plaice was found in the inshore water areas.These are the nursery areas. Spawning in the Irish sea stock takes place between January and April, peaking in late April to early March in several discrete spawning grounds. (Dunn and Paulson, 2002) When the eggs hatch, the larvae use the tide to drift to the shallower waters close to the shore, moving down the water column on the outgoing tide to avoid being swept back (McCloghire et al. 2004). These shallow waters are the preferred habitat for juvenile plaice and also important feeding grounds. Living on these sandy or muddy bottoms they feed on creatures such as polychaetes, small thin shelled molluscs and crustaceans (Froese et al. 2009) so it is not surprising that a large number of plaice are found here. As they grow older and larger they generally move further offshore. Different size and age classes tend to separate out spatially in the habitat (Metcalfe et al.2006). In heavily fished stock the juvenile fish tend to be the most abundant group in the stock (Longhurst, 2002) Most fish in this study are 2.5 years old. Considerably less fish were found offshore,only 850 fish offshore compared to 6245 caught inshore.
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Males and females show several differences in size /age frequencies, length /weight relationships, mortality rates, sex ratios and age/size at maturity. Females are generally slightly larger than the males in this species (Froese et al., 2009) However, females in the study are attaining significantly larger sizes and living longer than the males. Although both males and females reached maturity at a similar age and the females were only slightly larger at maturation, the females subsequently increase significantly more than males. Moreover, fish over 27cm in length are predominantly female. However, the largest class size found was only 42-46cm although these fish can grow to 100,0cm (Nielson, 1986). Body lengths of female fish have dropped throughout the twentieth century (Grift et al,2003). Changes in growth maturation and reproductive investment have been described in North Sea plaice ( Rijnsdorp, 1993a, Law and Grey1989).
Accurate fish population data and examination of fish population biology is important for fish management and the development of strategies to allow these resources to be exploited sustainably. Fisheries management techniques, such as mesh size regulations, attempt to maintain the stock sustainably by permitting smaller, younger fish to pass through the net and survive to produce the next breeding stock. has resulted in the capture and mortality of the larger fish in the population. Consequently fish that are left to breed are those that mature at a smaller size. This may be driving the observed reduction in size of fish within the population. Recent research (Deickman et al., 2007, Walsh et al,. 2006) suggests that the selective removal of larger, older fish may have negative effects on stock replenishment, inadvertantly reducing resilience and stock productivity.Although plaice can live to an age of 40 years (Clover 2004) the oldest fish caught in this survey were only 7.5 years old. Fisheries pressure on highly exploited populations cause fast growing fish to be caught sooner, giving them less time to reproduce and those fish that delay maturation may be caught before they reproduce with the consequence that the effects of fishing on mortality may be favouring slower growth, earlier maturation and higher reproductive investment. The possible effects of long term size and age truncation by removing stronger fish and leaving slower growing individuals has been replicated in experiments (Conover and Munch, 2002, Reznck et al.1993). This has implications for sustainability, suggesting that evolutionary processes may need to be considered in management strategies (Jorgensen et al. 2007).
The timing of maturation, around three years in this survey with 50% females mature at 32cm, and 50% males mature at 29.5cm, influences expected reproductive success. Larger females produce larger clutch sizes. Moreover, older fish generally have better reproductive success (Berkeley et al, 2004). In the absence of fishing pressure, delayed maturity is an advantage but not so if the risk of mortality is high.Changes in length at age when maturation probability reaches 50% may be indicative of fisheries induced evolution.(Dieckmann and Heino,2007, Heino et al 2002, Enando et al.2004 ) Studies of North Sea plaice (Dieckmann and Heino, 2007 )show a significant decrease in body length at 50% maturity. Irish Sea stocks may be showing similar trends. Asymptotic total lengths of 46 cm for female plaice and 36.3 cm for males caught off east Anglesey for the period 1974 to 1977 (Basimi et al. 1985) suggest a reduction in size of about 6-8 cm in the theoretical maximum length of the fish since the 1970s. Overfishing in the Irish Sea from the 1970s until the early 1990s may have caused this effect on the plaice population..
Models combining population and genetic data support theories of fisheries induced population change (Dunlop et al. 2009, Enberg et al.2009). Modelling can be a useful tool for analysing complex inter-relations of population dynamics and forecasting the effects of fishing management regimes.However, analyses of long term data cannot definitely prove evolutionary change. (Dieckmann et al. 2009). Other environmental factors can also effect population dynamics and need to be incorporated in models (Rjinsdorp and Leeuwen,1996). Pollution such as industrial waste and nutrient loading of coastal waters impact on the coastal zone, As plaice are a dermesel fish their life cycle is strongly dependent on bottom conditions. Recruitment of plaice has shown to be negatively effected by increased bottom covering of filamentous green algae on coastal sea beds (Paulson, 2008. Pihil et al, 1995 ).
Estimates of up to 60% adult dermesal fish removed in the Irish Sea through trawling in 1987 (Dickson,1987) imply a history of serious overfishing. From the early 1970s to the early 1990s stock exploitation exceeded precautionary fishing mortality rates. A reduction in landings from 3,300 tonnes to 600 tonnes from 1992 to 2008 and a marked increase in the spawning stock biomass up to 9000 tonnes in 2009 has been recorded (ICES, 2009). Although this appears to indicate a recovery of the stock in response to reduced fishing effort, comparable pre-exploitation figures do not exist. Fish population dynamics still appear to be affected by fishing activity. Changes in population structure, with earlier maturity, decline in size at maturity, and the loss of older age groups seems to characterise this and other European plaice populations that have been studied (Kuznetsova et al.2004, Bromley,2000, Rijnsdorp, 1989, Rijnsdorp and van Leeuwen,1996.Grift et al. 2003 and 2007, Nielson et al. 2004). More needs to known about spatial and geographical extent of populations and populations interactions. The strong tidal streams in the Irish Sea probably increase spatial distribution of juvenile fish (Mc Cloghire et al 2004). Electronic tagging of populations (Hunter et al.2003, Dunn and Paulson 2002, Arnold,2000) could aid in the development of better spatial management strategies. It is probable that climate change is also having an impact on the population dynamics. Statistical analyses needs to be combined with spacial and temporal distributions in relation to changes in growth patterns and environmental variables. In this way a fuller understanding of the population dynamics can be used to critically assess commercial fish management strategies in the Irish Sea.
Fig 1.Admiralty chart showing sample areas and trawl lines.
Fig 2 Age and lenght frequency distributions.
Fig 3. Male and female length weight relationships.
Fig. 4.Von Bertalanffy growth curves for males and females.
Fig. 5.Linear semi- logarithmic mortality for males and females.
Fig. 6. Male and female length classes at 50% mature.
Fig. 7. Male and female ages at 50% mature
Fig. 8. Mean total length of fish caught each year.
Table 1. Numbers of fish caught inshore and offshore.
Table 2. Numbers of males and females measured in each age class.
Table 3. Numbers of males and females in each age class.
Length class (cm)
Length class (cm)