浙江南部近海小黄鱼生长、死亡和单位补充量渔获量

浙江南部近海是东海种群小黄鱼(Larimichthys polyactis)的重要繁殖和育肥场所。根据2016年2月、5月、8月和11月采集的2023尾浙江南部近海小黄鱼全长、体长和体重等生物学信息,利用体长频率分布估算小黄鱼种群生长、死亡参数,并利用Beverton-Holt动态综合模型评估探讨单位补充量渔获量在不同自然死亡系数和渔具选择下随捕捞死亡系数的变化趋势。研究结果表明小黄鱼von Bertalanffy生长参数为渐近体长L_∞=22.58 cm,生长速率K=0.78/a,初始年龄t_a=-0.37 a;自然死亡系数M值为1.343,总死亡系数Z值为4.432,捕捞死亡系数F为3.089,开发率E为0.697,表明资源处于过度开发状态;小黄鱼的首次捕捞体长L_(50)=13.11cm,对应首次捕捞平均年龄t_c=0.743 a,小于临界年龄(0.886a)和体重生长的拐点年龄(0.979a),渔业主要捕捞对象为幼鱼和补充群体,无法保证资源的有效补充;根据B-H动态模型,当前的YPR值为15.279 g/ind,若降低捕捞强度到1.685,对应YPR_(max)为17.061 g/ind,与当前产量相比增幅11.66%;若提高开捕体长(13.11cm→16.0cm),YPR_(cur)会出现显著提高(15.279 g/ind→18.766 g/ind),增幅达22.82%,表明提高开捕体长要优于降低捕捞强度。当前东海小黄鱼群系处于小型低龄化和过度开发状态,建议将小黄鱼的开捕体长提高为15 cm,保证小黄鱼的产卵亲体量及资源的可持续发展。     

Growth trials of Crassostrea gigas and Ostrea edulis in inshore waters of Malta (Central Mediterranean)

To investigate the possibility of establishing oyster culture in Malta, growth trials with C. gigas and O. edulis were undertaken. The oysters were placed in floating cages at depths of 3 and 8 m at Marsaxlokk bay, Mistra bay and Rinella creek. These oysters were sampled at two-month intervals for their growth, meat-shell index and survival. The temperature, salinity, oxygen tension, chlorophyll-a levels and the levels of particulate matter in suspension in the water at the test sites were monitored concurrently. A programme of qualitative and quantitative analyses of the natural phytoplankton population was also carried out.O. edulis suffered catastrophic summer mortalities at all locations when the ambient temperature of the water reached about 25°C; this virtually rules out any possibility of starting any flat oyster industry using this system. As for C. gigas, Marsaxlokk bay 3 m appears to hold some promise for its culture. At this location, this species showed reasonable growth, relatively low mortality and normal shell growth. Although at Rinella overall growth of C. gigas was comparable to that at Marsaxlokk at 3 m, the Rinella oysters accumulated copper and exhibited abnormal and thickened shell growth with the result that the oysters were of a much inferior condition. Growth at Mistra (both depths) and Marsaxlokk 8 m was stunted.No clear correlation could be established between the performance of the oysters and the environmental parameters monitored. Several factors that may have been responsible for the observed differences in oyster performance are discussed. In particular, the nature of the water movements is thought to have played a key role in dictating the growth of the oysters.     

Growth and mortality of the narrowbarred Spanish Mackerel, Scomberomorus commerson (Lacepède), in Omani waters

Length composition data from quarterly catches of Omani narrowband Spanish mackerel, Scomberomorus commerson (Lacepède), landed between 1987 and 1995 were used to estimate the growth, mortality and exploitation parameters of the stock. Non-linear least square fitting provided a complete set of von Bertalanffy growth estimates: L _∞ = 173.6 cm fork length; K  = 0.28; and t ₀ = −0.86 years. There were no seasonal differences among the growth estimates. The instantaneous natural mortalities determined by three independent methods based on life-history parameters were 0.35, 0.64 and 0.77. Seasonal total mortalities were calculated by the length converted catch curve method for evaluating seasonal exploitation ratios ( E ). The E -values exceeded 0.4, indicating overexploitation during almost all the seasons studied. This was supported by the large reduction in the total landings of S . commerson from 27 762 t in 1988 to 3265 t in 1993.     

舟山近海黄姑鱼胚胎发育及前期仔鱼形态的观察

通过对黄姑鱼胚胎发育全过程的连续观察,描述了黄姑鱼胚胎发育的特征、发育速度及前期仔鱼的形态特征等。观察结果表明,黄姑鱼的胚胎发育与其它硬骨鱼浮性卵鱼类基本相似,属端黄卵,盘状分裂。在水温22.0~22.5℃、盐度26.5的条件下,黄姑鱼受精卵经24h15m in孵出仔鱼。初孵仔鱼全长1.75±0.09mm,孵出后约50h,卵黄囊完全被吸收。     

浙江南部海域蓝圆鲹生长异质性及死亡特征

蓝圆鲹（Decapterus maruadsi）属暖水性中上层鱼类，是中国近海重要的渔业资源。本文根据2015-2018年浙江南部海域（120.5°E~123.5°E，27°N~29°N）底拖网季度调查数据，运用线性混合效应模型等方法，研究了蓝圆鲹的叉长-体重关系和生长异质性，并估算其自然死亡系数。结果显示，蓝圆鲹样本叉长（L）范围为45.0~247.0 mm，平均叉长为126.1 mm；体重（W）范围为0.7~206.6 g，平均体重为29.1 g；叉长-体重关系式为：W=5.01×10^-6L^3.17。线性混合效应模型结果显示，同时考虑季节、性别和年份差异的模型对蓝圆鲹叉长-体重关系的拟合效果最佳。在相同叉长条件下，体重在秋季最大，其次是春季，而夏季最小；从不同年份来看，体重在2018年最大，其次是2016年，在2017年最小；从不同性别来看，雄性和雌性的差异不明显。这说明时间因素对于浙江南部海域蓝圆鲹的生长特征具有显著影响，2018年秋季蓝圆鲹的长势最佳。依据"Pauly"、"Pauly update"、"Jensen"、"Hoenig"和"Lorenzen"等不同的经验方程估算的蓝圆鲹自然死亡系数在0.36~1.41之间。本研究通过混合效应模型分析了年份、季节、性别对蓝圆鲹叉长-体重的影响，这对蓝圆鲹生活史特征及资源评估具有重要的参考意义。     

基于双频识别声纳(DIDSON)的辽东湾近海沙海蜇监测的应用研究

为了解双频识别声纳(DIDSON)在近海海域大型水母沙海蜇(Nemopilema nomurai)监测中的效果, 本研究于 2017 年 7 月、8 月通过 DIDSON 的走航观测对辽东湾近海伞径 16~81 cm (7 月份平均伞径 33.01 cm±13.17 cm, 8 月份平均伞径 57.03 cm±13.44 cm)的大型水母沙海蜇开展丰度和垂直分布的监测调查, 并同步结合传统网具调查, 以验证和比对 DIDSON 观测在大型水母沙海蜇监测中的效果。调查结果显示: DIDSON 观测影像可直观显示沙海蜇的数量及其在水层中的位置。7 月份 3 个调查断面声纳观测的沙海蜇丰度分别是 6.13 ind/1000 m3、6.71 ind/1000 m3、 7.93 ind/1000 m3, 平均丰度为(6.92±0.75) ind/1000 m3。8 月份 3 个调查断面声纳观测的沙海蜇丰度分别是 1.41 ind/1000 m3、 1.23 ind/1000 m3、0.55 ind/1000 m3,平均丰度为(1.07±0.37) ind/1000 m3。比对锚流网调查沙海蜇的丰度, 7 月份和 8 月份 DIDSON 监测的沙海蜇平均丰度分别是网具调查的 4.89 倍和 3.95 倍。声纳观测影像显示沙海蜇主要栖息在调查海域中上层水域(1.1~10 m), 底层水域沙海蜇出现的少。比起传统的网具调查, DIDSON 更加精准的监测调查到调查海区沙海蜇的丰度, 并观测到沙海蜇的垂直分布, 证明双频识别声纳可以适用于浅海大型水母沙海蜇的监测调查工作。研究结果表明, DIDSON 观测可以扩展大型水母沙海蜇监测的内容, 在对其行为学的观测上具有较大的优势。通过延长 DIDSON 走航式观测时间, 扩大 DIDSON 探测范围, 并结合传统网具, 可以更加精准的开展沙海蜇数量的监测。     

Growth and mortality of cultured bay scallops in the Damariscotta River, Maine (USA)

Growth and mortality of hatchery-reared bay scallops, Argopecten irradians, were determined over an 18 month period (July 1994-December 1995) at two locations in the Damariscotta River, Maine, north of its natural distribution. Growth occurred primarily between May and October when mean water temperature exceeded 10 °C. Final mean shell height (56.3 mm) was significantly greater (p 0.0005) at the upriver site than at the downriver site in association with a higher mean water temperature. Cumulative mortality (43-49%) over this period occurred primarily over the winter months, in conjunction with mean water temperatures below 5 °C. Although it is possible to rear bay scallops to commercial size in Maine, the short growing season, high winter mortality, and costs associated with over-wintering and fouling will likely deter commercial culture.     

Fecundity and growth-dependent mortality of Pacific anchovy (Engraulis japonicus) in Korean coastal waters

We propose a method of estimating natural mortality of marine pelagic fishes, especially for early-life stages, based on their fecundity. To estimate size-dependent fecundity, growth and mortality of Pacific anchovy (Engraulis japonicus), the most abundant fish species in coastal waters off the Korean peninsula, we undertook a synthesis of results from past studies and data. Assuming that the growth coefficient K varies with water temperature, we derived a modified von Bertalanffy growth equation covering all life stages based on otolith analysis of anchovies collected from southwestern coastal area of Korea in 1996. By revisting a past study on spawning and egg production of anchovies in the southern Korean coastal waters, we calculated a monthly-averaged fraction of mature females spawning per day to estimate that an average female anchovy spawns 36 times per year, and that the mean number of eggs produced by an average female is ca. 160 × 10³ yr⁻¹. Accepting the ‘bigger-is-better’ hypothesis, we derived a theoretical mortality curve that assumes instantaneous natural mortality as an inverse function of anchovy body length. Assuming equilibrium status of stock, estimated annual instantaneous mortality of anchovy between egg to age-1 stage was 11.3 yr⁻¹ and estimated size-specific mortality was 1.24 d⁻¹ mm in fork length. The derived theoretical mortality curve fit well the stage-specific mortalities, which were estimated independently based on ichthyoplankton surveys and anchovy samples collected by commercial nets, but underestimated the egg mortality (0.89 d⁻¹ vs. 0.83 d⁻¹).     

Growth,mortality and reproductive traits of diploid and triploid Pacific oysters (Crassostrea gigas,THUNBERG, 1793) in Southern Brazil

This work aimed to evaluate the whole weight (g), shell height (mm), mortality (%) and reproductive aspects (condition index and histology) of triploid (3N) Pacific oysters (Crassostrea gigas) cultivated in southern Brazil. To accomplish this, a comparative experiment was performed with a diploid (2N) control group in two cultivation sites on Santa Catarina Island: North Bay (NB) and South Bay (SB). Whole weight, height and cumulative mortality were evaluated monthly for eight months and condition index (CI) for six months. Histological analysis was performed every two weeks for four months to investigate reproductive aspects. At the end of the experiment, whole weight and height were similar for 3N and 2N oysters. However, while more growth in whole weight and height was observed in the initial phase of cultivation in NB, such growth was not detected in SB until intermediate and final grow‐out phase. At harvest, cumulative mortality in 3N oysters was similar that in 2N oysters. The results suggest that diploid and triploid oysters grown in southern Brazil have similar developmental patterns based on whole weight and height, as well as reproductive cycle. In addition, cultivation sites have more influence on growth and mortality than ploidy levels.     

黄海南部小黄鱼生长、死亡和最适开捕体长

根据2008-2009年在黄海南部海域底拖网采集的小黄鱼(Larimichthys polyactis(Bleeker,1877)]样本的生物学资料,应用FiSAT软件对小黄鱼的生长、死亡参数进行估算,通过Beverton-Holt模型评价小黄鱼资源的利用状况,并与临界体长、拐点体长等进行比较,以确定最适开捕体长.结果显示:(1)小黄鱼的体长(L)和体质量(W)关系式为W=0.0268×L2814.(2)应用FiSAT软件拟合的von Bertalanffy生长方程参数为L=27 cm,k=0.45 a-1,to=-0.47 a;(3)总死亡系数Z=2.40,自然死亡系数M=0.77,捕捞死亡系数F=1.63,开发率E=0.68.(4)在现行的捕捞死亡系数F=1.63下,Yw/R达到最大值时开捕年龄和开捕体长分别为1.41 a和15.42 cm;拐点年龄和体长分别为1.83 a和17.41 cm;临界年龄和体长分别为1.70 a和16.82cm.现阶段小黄鱼资源已处于过度开发状态,综合考虑渔民利益和资源修复需要,建议黄海南部小黄鱼的最适开捕规格定为体长14.83 cm.     

Growth,mortality and reproduction of the transplanted Manila clam (Ruditapes philippinarum Adams & Reeve 1850) in Jiaozhou Bay

Samples of Manila clam (Ruditapes philippinarum Adams & Reeve 1850) were collected from May 2004 to April 2005 monthly, and plankton net trawling of planktonic larvae and bottom sediment sampling surveys were further conducted from May to October 2006 in Jiaozhou Bay. Based on the data collected, growth, mortality and reproduction of the transplanted Manila clam and the environmental effects were examined. The results showed that the enhanced clams grew well and showed a growth trend similar to the local wild ones. The main growth periods lasted from April to September, with the water temperature being the main factor affecting the growth, which was the same as that of the wild clams. There were also two reproduction cycles for the farmed Manila clams each year in Jiaozhou Bay and the main breeding period was from May to June. The phenomenon of delayed metamorphosis was quite common through larval development. The farmed clams could spawn when they reached sexual maturity, but they could not perform effective recruitment as many planktonic larvae died during metamorphosis and settlement. A preliminary study indicated that sediment perturbation and marine environment pollution were the main factors causing the death of larvae in the development process.     

Growth of juvenile common octopus (Octopus vulgaris) in Korean waters is affected by shelter type

This study evaluated the effects of shelter pipes (PS), tube shelters (TS), and no shelter (NS) on the growth of juvenile common octopus (Octopus vulgaris). The grow-out was studied in situ by conducting a passive follow-up survey using integrated transponder tags. The growth and survival rates were evaluated in two experiments. Each experiment included two replicates. Both experiments ran for 60 days, for which each trial included 24 octopuses that were fed once a day a diet of frozen sardines (Sardinops melanostictus) and swimming crabs (Portunus trituberculatus), at 3–8% body weight. We investigated the relationship between the beak microstructure of the common octopus and possible stress events caused by culture conditions at 15 °C for 15 days. The nine juveniles were separated into three groups (NC: natural conditions, PS: culture conditions as a group, and TS: culture conditions with individuals kept separately) to analyze the amount of stress caused by rearing conditions. Stress marks were observed using a high-resolution stereo fluorescence microscope. In Experiment 1, the specific growth rates (SGRs) were 0.20 ± 0.15% per day in the PS group, 0.24 ± 0.10% per day in the TS group, and 0.68 ± 0.48% per day in the NS group. The survival rate was 62.5 ± 6.25%, 37.5 ± 18.75%, and 12.5% in the TS, PS, and NS groups, respectively. The experimental conditions of Experiment 2 were the same as those in Experiment 1, except that Experiment 2 involved different individuals and occurred during a different season. In Experiment 2, the SGRs were 0.54 ± 0.20% per day in the PS group, 0.59 ± 0.22% per day in the TS group, and 0.44 ± 0.10% per day in the NS group. Survival rate was 87.5 ± 6.25%, 75 ± 6.25%, and 75% in the TS, PS, and NS groups, respectively. Regression analysis showed that the water temperature (from 5.4–23.1 °C) affected the growth of octopuses in Experiments 1 and 2. Capture stress was detected in six of the nine individuals (NC, PS, and TS combined). Statistically significant differences in the stress levels were detected between the PS and TS groups (P ≤ 0.1), with the NC group exhibiting no stress lines after initial adaptation, unlike PS. In conclusion, octopuses kept in TS were not affected by stress after initial adaptation.     

东、黄海星康吉鳗生长、死亡和单位补充量渔获量

依据2011年、2016年和2017年在东、黄海进行的底拖网调查数据,研究了星康吉鳗的生长参数、死亡系数和单位补充量渔获量(YPR)。星康吉鳗的体长体质量关系和生长方程的拟合结果表明,其条件因子a估计值为4.5×10–4,其异速生长系数b的估计值为3.3。其渐近体长L∞的估计均值为102 cm,生长速率K的均值为0.21/年,理论上体长为零时的年龄t0估计均值为–0.19。通过Pauly经验公式求得星康吉鳗的自然死亡系数(M)为0.33。体长转换的渔获曲线求得总死亡系数为3.36,进而求得现阶段的捕捞死亡系数(F)为3.03,开发率高达90%。此外求得现阶段星康吉鳗的开捕体长为30 cm,对应的开捕年龄为1.47龄,远小于其体质量生长的拐点年龄(3.70龄)和临界年龄(3.39龄)。本研究根据单位补充量渔获量模型,估计其在不同自然死亡系数和开捕体长的情况下随捕捞死亡系数的变化曲线,并估算其生物学参考点。随F增加,YPR先增加到最大值,再逐渐减小。现阶段星康吉鳗的YPR为27.14,而当M为0.33时Fmax估计值为0.38,YPRmax为52.89,即将F降低为1/8,可获得将近2倍的YPR;对应的F0.1为0.255,比Fmax降低了三分之一,YPR0.1(50.38)只比最大值降低了不足5%。M越大,求得的生物学参考点越大,可获得的YPR越小;当开捕体长增加时,F和YPR都会有不同程度的增加。因此,现阶段中国东黄海星康吉鳗的开发率过高,开捕体长过小,处于过度捕捞的状态。为了维持星康吉鳗种群的生态健康、实现该渔业的可持续发展,建议大幅削减其捕捞强度,降低捕捞死亡系数,同时增加开捕体长。     

基于海洋遥感的西北印度洋鸢乌贼渔场形成机制的初步分析

为了解西北印度洋公海中阿拉伯海外海的渔业资源开发状况、探究鸢乌贼 (Symplectoteuthis oualaniensis) 的渔场分布及其渔场环境要素对资源的影响,对阿拉伯海外海开展了2个航次的海上调查,获取每个调查站点的渔获量、水温等数据,利用广义可加模型 (Generalized Additive Model, GAM) 分析了多种海洋环境与时空要素对该海域鸢乌贼单位捕捞努力量渔获量 (Catch per unit effort, CPUE) 的影响。研究发现：1) 鸢乌贼CPUE高值的集中区域主要有3个 (60°E—61°E, 16°N; 62°E, 16°N—18°N; 61.50°E—63°E, 17°N—17.75°N);2) 最佳GAM模型的影响因素为经纬度、农历日、海表温度 (Sea surface temperature, SST)、叶绿素a (Chl a) 浓度和海表温度距平 (Sea surface temperature anomaly, SSTA),其中方差解释率最高的前3位分别为经纬度、农历日和SSTA;3) 农历日要素统计得到月黑夜总作业次数是月亮夜总作业次数的1.38倍,月黑夜的总渔获量比月亮夜的增加108.21%;4) GAM模型表明最适SST介于26.5~27.5 ℃,Chl a浓度介于0.30~0.50 mg·m⁻³,SSTA介于0~0.4 ℃。     

温台渔场龙头鱼的生长、死亡及最适开捕规格

在传统渔业资源衰退的情况下,龙头鱼等次要的经济种类逐渐成为东海海区的优势种群,具有重要的经济价值和生态地位。根据2015年11月—2016年8月在温州台州外海(120.93°~122.95°E,27.21°~28.97°N)调查采集的2 611尾龙头鱼的生物学数据,用ELEFANⅠ方法对其生长、死亡参数进行估算,并通过不完全β函数渔获量方程动态综合模型确定最适开捕规格,评估种群资源的利用状况。结果显示,温台渔场龙头鱼体长—体质量关系为W=0.000 5L3.87(R2=0.906 7),雌、雄个体间的体长—体质量关系无显著性差异;龙头鱼的Von Bertalanffy生长方程参数L∞、K、t0分别为32.13 cm、0.39和–0.69龄,体质量生长拐点年龄为2.78龄;根据体长组成资料的线性渔获量曲线估算出龙头鱼的总死亡系数2.55,用Pauly经验公式估算出自然死亡系数0.66,捕捞死亡系数1.89和开发率0.74;动态综合模型评估表明,应适当提高开捕年龄,增大开捕体长,最适开捕年龄tc为1.15龄,相应开捕体长为16.5 cm。     

Reproductive cycle of the rock oyster,Striostrea prismatica (Gray, 1825) from two locations on the southern coast of Ecuador

The reproductive cycle of the rock oyster Striostrea prismatica was determined at two fishing areas, General Villamil and Ayangue (located on the southern coast of Ecuador), between May 2012 and April 2013. Monthly sampling campaigns were performed at both locations. The tissues were histologically examined to determine gonadal index (GI), oocyte development, follicular area coverage and sex ratio. Surface seawater temperature, salinity and chlorophyll a concentration were measured during samplings. Our results show a similar annual reproductive pattern at both locations. The GI reached maximum values during the summer. Oysters reached highest ripeness in January and February, while spawning occurred in February–March. Gametogenesis was linked to a consistently increasing follicular area (from 1.3 ± 0.7% to 85.8 ± 7.8%) and associated to surface seawater temperature. Spawning coincided with warm water temperature fluctuations and a seawater salinity decrease. No correlation was found with Chlorophyll a concentration. The sex ratio of sampled populations was 1:1, suggesting that oysters sizing more than 10 cm in shell length present a stable sex proportion in the population. The diameter of mature oocytes was significantly reduced (32.7%) during histologically preparations in comparison to fresh oocytes. Our study provides useful information of environmental factors that may control the observed gametogenesis and spawning activity of S. prismatica.     

Growth,sexual maturation,and spawning season of Atlantic halibut,Hippoglossus hippoglossus,in Faroese waters

Biological data on Atlantic halibut, Hippoglossus hippoglossus, caught in an exploratory bottom trawl fishery in November–February 1983–1986 in a recently discovered deep water spawning area and in other both coastal and bank areas in Faroese waters, are presented.Studies of increases in length and weight confirmed the sex-dependent growth patterns found in earlier works, i.e. a higher growth rate in females than males especially after sexual maturity. The data show that Faroese halibut grow much more rapidly and mature at lower average ages (c. 4.5 years in males and slightly above 7 years in females) than in any other area investigated in the North Atlantic. Average length and round weight at sexual maturity were slightly above 55 cm and c. 1.7 kg in males, while the corresponding values in females were 110–115 cm and c. 18 kg.Spawning in the area appeared to commence in January and intensify in February. The peak in spawning activity was apparently not attained during the study period (November–February), and most probably occurred later in winter or early in spring.     

Seasonal movements of silver kob, Argyrosomus inodorus, (Griffiths and Heemstra) in Namibian waters

A seasonal migration pattern for silver kob, Argyrosomus inodorus (Griffiths & Heemstra) is elucidated based on tag-recapture data. Spawning adults start migrating southwards against the north-westerly surface currents at the beginning of the austral summer from the northern end of their distributional range to their spawning grounds, Sandwich and Meob Bays, at the southern end of their distributional range. It is suggested that after spawning larvae drift north with the current to the nursery areas, the West coast Recreational area (WCR). When juveniles reach the age of approximately 2 years they will gradually move north towards the adult feeding ground, the Skeleton Coast Park (SCP) waters. At the end of the spawning season when the surf-zone water temperature decreases to about 15 °C adult silver kob complete their spawning cycle by returning to the adult feeding ground, probably moving slightly offshore and with the current.     

Growth and mortality of juvenile sunfishes (Lepomis sp.) under heavy predation

The relationship among growth, energy storage and mortality of juvenile bluegills (Lepomis macrochirus, Rafinesque) experiencing heavy predation was experimentally investigated in a large reservoir where piscivorous largemouth bass (Micropterus salmoides, Lacépède) were abundant. I conducted experiments evaluating interspecific competition, density-dependent growth and lipid content and habitat-specific growth and mortality rates of juvenile bluegills in this reservoir. Enclosure experiments showed that the growth rates of bluegills were unaffected by presence of other Lepomis species but decreased with increasing bluegill density. However, at densities corresponding to those found in the reservoir, the growth rates of bluegills were high, and competition appeared to be relatively unimportant. The percentage lipid content of bluegills was unaffected by the presence of other species, density of bluegills or habitat. The growth rates of bluegills in vegetated refuge areas were significantly higher and mortality rates were significantly lower compared with open water habitats. The low densities and high growth rates of juvenile bluegills in refuge environments suggests that there is a strong lethal effect of predators on this population of bluegills.