European plaice (Scientific name: Pleuronectes platessa) or hereby known as plaice is a demersal species of fish and is an important fish for Northern Europe fisheries. It is widely distributed in the waters from Western Mediterranean and the White Sea. Through different statistical data, it can be noted that the total amount of landing of this particular species is about 594 tonnes annually. For the cause of great economic value of this particular species, different management strategies have been implemented for the purpose of prevention of the stocks from collapse because of the advances in technology and historical events such as climate (Ciotti et al. 2014). The outcome has been a varied exploitation pattern, potential yield of fish stocks and sizes in the population (Cardinale et al. 2010). It should be also kept in mind that if a management plan is needed to be successful, the total population dynamics of this particular species of fish is assessed for long-term sustainable exploitation. In terms of industrial prospect, the Irish Sea region is considered as one of the most important single stocks along with the Cardigan Bay, St. George’s Channel, The Bristol Channel and the Celtic Sea (Deadman et al. 2015).
The sustainability measurement is based on the three primary parameters which are, growth, mortality and recruitment of the fish with respect to fisheries industry. It is important to record such data as the population decrease will result in low levels of reproduction and the whole fisheries industry will face a significant setback (Toonen and Mol 2013). Another aspect, which should be considered as if the numerical of this particular species decrease, then there, is a huge possibility that the species can become endangered as well (Marx 2013). There, it is important to monitor the biological parameter on a regular basis for the purpose of conservation and reservation of this particular species based on age, size, sex and group (Liedtke et al. 2013). With respect to this context, the exploitation should also be considered as it is considered one of the major issues in current scenario.
Though there is presence of some management systems for the plaice stocks in the region of Irish Sea, the species have been heavily fished in the United Kingdom region. As a result, the phenomenon of over exploitation is taking place. Therefore, it is very crucial to monitor the stocks (Caveen et al. 2014).
Plaice is mostly studied in flatfishes and is based on CPUE from Danish seine fleets that can be supplemented from ports of all countries. Annual trawl surveys have been taken continuously to give valuable amount of the stock size of the species. Since, 1986 MRI (Marine Reserach Institute) started a program based on sampling to keep a check on the plaice catches which are recorded in the logbooks since 1987 (Rotander et al. 2012). It is a known fact that over exploitation is casing removal of different marine lives across the globe which poses a threat for the endangered species to be permanently extinct. From studies, it can be noted that the human population exploit more than 400 species of marine lives for the purpose of food. With respect to this particular aspect, it can be also highlighted that along with fishing, ‘ghost fishing’, tropic cascading effects and food we competition are also harming the stock population of the Plaice species (Gibson et al. 2014).
The primary focus of this particular study is to measure and assesses all the biological parameters of plaice population present at the regions of Eastern-Anglesey and North-West Wales coastal water regions on growth, mortality and maturity. With respect to the ongoing studies, it can be said that a detailed data will provide important information in terms of fisheries science. The obtained result will provide information regarding the current amount of stock, which will help to design effective and efficient management strategies for the purpose of sustainable fishing (Pauly et al. 2013). Moreover, the study will outline the specifications of European Plaice that will be analysed from 2006-2015 to generate the effectiveness of current situation for implementing useful strategies.
Materials and Methods
The given samples were collected from the surveys on floorboard namely RV Prince Mandog, the known research pot in Bangor University for the School of Ocean Sciences. The surveys were carried from the year 2006 to 2015 for 10 years, with two supplementary search taking place in December of 2010 and 2011. The data sites included were same as included earlier. They were namely “Conwy Bay” and “Red Wharf Bay”, the inshore sites and “Colwyn Bay” and “Point Lynas”, the offshore sites. The outer search was taken with the help of mesh size of 76mm. The check lasted for a middling 57 minutes. The sorting of catch was done with the measurement of each fish to the nearest value of mm and then prearranged into strata of length subclass. The subclass limited to 1cm size class amongst the first three fishes, which were gradually retained for further analysis and interpretation.
After examining and analysing the fishes, each fish was documented under the total length (cm) and weight (g). The fish was classified into different categories right from age to sex with specification of examined gonads to study the maturity level of each fish. The sagittal otoliths of every fish were separated and were simultaneously positioned in the labelled envelopes. Subsequent to separation of otolith, the fishes were histoclear and were viewed under a stereo microscope for the determination of ages. The maturity categorization were according to three stages identified as Stage 1 as immature, Stage 2 as maturing and Stage 3 as mature. Age frequency was determined to 0.5 years according to the plaices under the microscopic examination.
Profusion and distribution of frequency
The data from each site taken was initially summed to find the total number plaice, and later was divided and alienated by the drag time to discover the per unit catch effort (CPUE). Each plaice’s total length (TL) was alienated to lessen variability into 3 cm size divisions. The age, size and class was calculated according to the percentage of each plaice trapped.
For structuring growth, each plaice was studied based on the affiliation amid length and weight for females and males individually. The results were generated using the following equation W= aLb (where W is weight in grams, TL is the average in centimetre and a and b are stable variables) (Wang et al. 2015). The slopes of female and male regression lines were carried out by performing linear regression and general linear model in SPSS statistical tool for the efficiency of p value. The regression lines were calculated to institute whether there is any difference between the sexes. However, to investigate further, the growth of plaice one-sample t test was constructed to find whether the growth of plaice is isometric or allometric (Langan 2012).
The non-linear regression model was performed individually on male and female samples to examine the von Bertalanffy growth equation, Lt=L? [1-e-k (t-to)] (where Lt stand for the average TL at age t (years), L? is the greatest length, k is the Brody coefficient of growth (year-1), and t0 symbolizes the notional age at duration zero (year)). These likelihood ratio tests were subject to inspect the significant differences between the female and males growth curves and whether is efficient in nature. (Marriott et al. 2012).
The frequency calculated by gender, age and size of plaice with the population from 2006 to 2015 was calculated based on the chi square to show the differences were significantly different or not and goodness of fit was checked on R2 to reveal tghe significances of the frequencies and the population of plaices. The equation Y=1/[1+e-r(L-L50)] (where, Y is the percentage of per length fully grown fish class (cm), L50 was the average taken for length, and r being stable), was enhanced to approximate the length of the plaice at 50% maturity (Beckensteiner 2013). The same method was used to approximate the age using SPSS statistical tool at 50% development of the plaice. SPSS results were designed to formulate to fit the model with the bent regression lines in the scenarios. The mean age was assessed as (A50) and length (L50) with the logistic equation, P=1/(1+exp[X-X50)](P is the fraction mature which is considered a particular age or length for class, X50 is the prescribed extent or at 50% maturity and r is stable variablewhere the maturity of plaice could be identified. The results are generated using SPSS and Excel results.
The mortality grades were considered using the collective female and male statistics. This poiunt refers to the commercial fisheries were the difference is not taken between the two (Marriot et al. 2012). The equation, M=-LN(0.01)/tmax was used to calculate approximately natural mortality of the plaice. The equation described, Nt=N0e-Zt was calculated for the entire mortality of the stock given by Z. Z-M then helped in calculating the mortality of fishing. The percentage form was expressed using the exploitation rate F/Z (Mohammadikia et al. 2014).
Profusion and distribution of frequency
A large amount of trawls were assumed between 2006 and 2015, during which 4974 plaice were retained for the subsample, of which 2330 were male and 2519 were female. 50.64% of Plaice trapped were from inshore sites and 49.36% from offshore sites as the location is shown in figure 1.
The range of length between female and male were highly variable. The range for female TL varied from 8.5 cm to 51 cm and the range of male TL varied from 6 cm to 37.3 cm is shown table 1. However, the majority female lengths were found amid 27 cm and 33 cm (consisting of 26.67% of the total females trapped), whilst the majority male lengths were found between 18 cm and 23 cm (consisting of 42.31% of the total males trapped).
The assortment was from 0 to 8 years in female fishes and in males it appeared to have less years, approximately from 0 to 5 years when compared with female fish. The preponderance of female fish trapped were 3 years old (35.84% of the females trapped) while males fishes results showed to be 2 years old (63.94% of male plaice trapped). This can be seen in table 2.
The general linear model under regression not only revealed noteworthy dissimilarities between the gradients but also the weight and length data for females and males that came out to be F2518 = 98234.353, P2328 = 72264.872, P3.240 (SE = 0.14424 R2 = 0.975, P < 0.0001 and for males, W = 391.430L3.024 (SE = .14162, R2 = 0.969, P < 0.0001 (table 3). The b-values were also appeared to be considerably different and thus made known allometric growth in the plaice (?™€; t = 268.821, 2328 d.f., P < 0.0000: ?™‚; t = 313.424, 2517 d.f., P < 0.0001) (table 3).
A probability ratio test has shown that major distinctions were highlighted amid development of male and female plaice regarding age (x2 = 59.786, d.f. = 80, P = <0.0001) (table 5). The growth curves of Von Bertalanffy for individual male and female can be instituted in figure 2 and can be portrayed for female as the following model Lt = 41.090 (1-e-0.376(t-0.410)) and for males as Lt = 30.031 (1-e-0.447(t-0.779)) (figure 3 and table 4).
The plaice trapped showed results as 37.3% of the females and 23.56% of the males, which were mature. Table 4 represents the proportion of mature plaice catch at per size class (cm). The values A50 and L50 for mutually between males and females were contrasting as (L50 of females was 7.672 cm and males was 40 cm. A50 for females was 3.4 years and males was 3.6 years.) For the genders of plaice united, the L50 value was 23.836 cm, and A50 3.4 years. It can be shown in figure 4.
The ?2 (chi square) values showed that size had no influence on the sex proportions such that the interaction came out to be negligible. The value of male fish came out to be Z=0.541year-1 and female fish came out to be as Z=0.341year-1. The total mortality was premeditated to be Z = 0.441 year-1.
There is no surprise that GLM shows that there is difference but not significant difference in the Z-values for male and female Red Gurnard. The p-values according to age were P < 0.001 is found in table 5 and figure 5.
The equation of growth curve Von Bertalanffy curve exposed that males and females possess distinct growth patterns individually. Male fish had utmost 6 years, when compared with females had maximum age till 8 years (Sch?ckel et al. 2012). As per the results, it is shown that female haves have more growth and age advantage as compared to male plaice. This growth rate depicted in results is very common in plaice (Lee et al. 2013). The males sufficiently use extensive physical reserves and energy for replication process than females that is why they achieve their maturity before females. Moreover, male plaice do not use much of their energy in growth in relation to females because they use excessive energy in reproduction than in growth size. At later stage, maturity in female plaice helps them to achieve a prominent stage where maturity allows females to reach a point where egg production could be prominently done (Nissling et al. 2013).
According to the earlier studies, it was shown that males achieve their maturity level in immature age before female plaice and that is primary reason for them to be in shorter in length. However, female and male in this investigation were found to be in accord with the preponderance of mature males between 18-23cm in contrast to mature females, which were between 33 and 38 cm. In addition, males were to gain maturity at 2 years and female at 3 years. The egg production in females necessitates more physical resources than males as stated earlier. After analysing the results, it can be said this is the prime reason for the difference between age and maturity in male and female plaice and length at maturity gives increased resources that support production of eggs in females (Ganias et al. 2014). Hence, the male fish does not increase in size because all the physical resources are exhausted earlier because of achieving earlier maturity.
The length and age varies drastically between different stocks of plaice as a result, it makes comparisons quite difficult in difference between the years.
For this learning, 50% maturity of length was 37 cm for females, and 30 cm for males. According to growth studies based on plaice, one recent study highlights the population based growth estimates of plaice showing that 0-group plaice is based on otolith analysis and DEB modelling. The maturity and growth are different through years with different patterns of temperature. However, according to study analysed and recent study in 2016 highlights that growth decline does not relates to size of the fish. However, according to data analysed from 2006 to 2015, shows an change but showed a smaller growth and not wholly dependent on size (Cardoso et al. 2016).
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