Biology and population dynamics of cowcod (Sebastes levis) in the southern California Bight

Cowcod (Sebastes levis) is a large (100-cm-FL), long-lived (maximum observed age 55 yr) demersal rockfish taken in multispecies commercial and recreational fisheries off southern and central California. It lives at 20–500 m depth: adults (>44 cm TL) inhabit rocky areas at 90–300 m and juveniles inhabit fine sand and clay at 40–100 m. Both sexes have similar growth and maturity. Both sexes recruit to the fishery before reaching full maturity. Based on age and growth data, the natural mortality rate is about M =0.055/yr, but the estimate is uncertain. Biomass, recruitment, and mortality during 1951–98 were estimated in a delay-difference model with catch data and abundance indices. The same model gave less precise estimates for 1916–50 based on catch data and assumptions about virgin biomass and recruitment such as used in stock reduction analysis. Abundance indices, based on rare event data, included a habitat-area–weighted index of recreational catch per unit of fishing effort (CPUE index values were 0.003–0.07 fish per angler hour), a standardized index of proportion of positive tows in CalCOFI ichthyoplankton survey data (binomial errors, 0–13% positive tows/yr), and proportion of positive tows for juveniles in bottom trawl surveys (binomial errors, 0–30% positive tows/yr). Cowcod are overfished in the southern California Bight; biomass during the 1998 season was about 7% of the virgin level and recent catches have been near 20 metric tons (t)/yr. Projections based on recent recruitment levels indicate that biomass will decline at catch levels > 5 t/yr. Trend data indicate that recruitment will be poor in the near future. Recreational fishing effort in deep water has increased and has become more effective for catching cowcod. Areas with relatively high catch rates for cowcod are fewer and are farther offshore. Cowcod die after capture and cannot be released alive. Two areas recently closed to bottom fishing will help rebuild the cowcod stock.

Migration patterns of young Pacific bluefin tuna (Thunnus orientalis) determined with archival tags

We investigated the migration and behavior of young Pacific bluefin tuna (Thunnus orientalis) using archival tags that measure environmental variables, record them in memory, and estimate daily geographical locations using measured light levels. Swimming depth, ambient water temperature, and feeding are described in a companion paper. Errors of the tag location estimates that could be checked were –0.54° ±0.75° (mean ±SD) in longitude and –0.12° ±3.06° in latitude. Latitude, estimated automatically by the tag, was problematic, but latitude, estimated by comparing recorded sea-surface temperatures with a map of sea-surface temperature, was satisfactory. We concluded that the archival tag is a reliable tool for estimating location on a scale of about one degree, which is sufficient for a bluefin tuna migration study. After release, tagged fish showed a normal swimming behavioral pattern within one day and normal feeding frequency within one month. In addition, fish with an archival tag maintained weight-at-length similar to that of wild fish; however, their growth rate was less than that of wild fish. Of 166 fish released in the East China Sea with implanted archival tags, 30 were recovered, including one that migrated across the Pacific Ocean. Migration of young Pacific bluefin tuna appears to consist of two phases: a residency phase comprising more than 80% of all days, and a traveling phase. An individual young Pacific bluefin tuna was observed to cover 7600 km in one traveling phase that lasted more than two months (part of this phase was a trans-Pacific migration completed within two months). Many features of behavior in the traveling phase were similar to those in the residency phase; however the temperature difference between viscera and ambient temperature was larger, feeding was slightly more frequent, and dives to deeper water were more frequent.

Swimming depth, ambient water temperature preference, and feeding frequency of young Pacific bluefin tuna (Thunnus orientalis) determined with archival tags

We investigated the migration and behavior of young Pacific Bluefin tuna (Thunnus orientalis) using archival tags. The archival tag measures environmental variables, records them in its memory, and estimates daily geographical locations based on measured light levels. Of 166 archival tags implanted in Pacific bluefin tuna that were released at the northeastern end of the East China Sea from 1995 to 1997, 30 tags were recovered, including one from a fish that migrated across the Pacific. This article describes swimming depth, ambient water temperature, and feeding frequency of young Pacific bluefin tuna based on retrieved data. Tag performance, effect of the tag on the fish, and horizontal movements of the species are described in another paper. Young Pacific bluefin tuna swim mainly in the mixed layer, usually near the sea surface, and swim in deeper water in daytime than at nighttime. They also exhibit a pattern of depth changes, corresponding to sunrise and sunset, apparently to avoid a specific low light level. The archival tags recorded temperature changes in viscera that appear to be caused by feeding, and those changes indicate that young Pacific bluefin tuna commonly feed at dawn and in the daytime, but rarely at dusk or at night. Water temperature restricts their distribution, as indicated by changes in their vertical distribution with the seasonal change in depth of the thermocline and by the fact that their horizontal distribution is in most cases confined to water in the temperature range of 14−20°C.

The effects of docks on seagrasses, with particular emphasis on the threatened seagrass, Halophila johnsonii

In March of 2005, the National Oceanic and Atmospheric Administration's Special Projects Office released "Population Trends along the Coastal United States: 1980-2008." This report includes population changes and trends between 1980 and 2003 and projected changes in coastal populations by 2008. Given the findings, pressure on coastal resources around the country will continue to rise, particularly in Florida. ... One of our most valuable coastal resources is seagrass, but human desire and need to live on the coast means that our habitat overlaps with suitable seagrass habitat. Seagrasses can be found in coastal areas around the world but are limited to relatively shallow, relatively clear water because of their reliance on light for photosynthesis. Seagrasses provide food for both small and large marine organisms, larval and adult stage. They provide shelter and habitat to a variety of commercially important fish and invertebrates. They baffle the water column and inhibit the resuspension of sediments. They prevent erosion and fix and recycle nutrients. The physical and ecological benefits of seagrasses make them very important to human welfare, but their light-limited coastal distribution makes them highly susceptible to anthropogenic influences.

The physiological effects of multiple forced submergences in loggerhead sea turtles (Caretta caretta)

Sea turtles are subjected to involuntary submergence and potential mortality due to incidental capture by the commercial shrimp fishing industry. Despite implementation of turtle excluder devices (TEDs) to reduce at-sea mortality, dead stranded turtles continue to be found in near-record numbers along the coasts of the western Atlantic Ocean and northern Gulf of Mexico. Although this mortality may be due to an increase in the number of turtles available to strand, one alternative explanation is that sea turtles are repetitively submerged (as one fishing vessel follows the path of another) in legal TEDs. In the present study, laboratory and field investigations were undertaken to examine the physiological effects of multiple submergence of loggerhead sea turtles (Caretta caretta). Turtles in the laboratory study were confined during the submersion episodes, whereas under field conditions, turtles were released directly into TED-equipped commercial fishing nets. Under laboratory and field conditions, pre- and postsubmergence blood samples were collected from turtles submerged three times at 7.5 min per episode with an in-water rest interval of 10, 42, or 180 min between submergences. Analyses of pre- and postsubmergence blood samples revealed that the initial submergence produced a severe and pronounced metabolic and respiratory acidosis in all turtles. Successive submergences produced significant changes in blood pH, Pco2, and lactate, although the magnitude of the acid-base imbalance was substantially reduced as the number of submergences increased. In addition, increasing the interval between successive submergences permitted greater recovery of blood homeostasis. No turtles died during these studies. Taken together, these data suggest that repetitive sub-mergence of sea turtles in TEDs would not significantly affect their survival potential provided that the animal has an adequate rest interval at the surface between successive submergences.

Tag-reporting levels for red drum (Sciaenops ocellatus) caught by anglers in South Carolina and Georgia estuaries

A total of 1784 legal-size (≥356 mm TL) hatchery-produced red drum (Sciaenops ocellatus) were tagged and released to estimate tag-reporting levels of recreational anglers in South Carolina (SC) and Georgia (GA). Twelve groups of legal-size fish (~150 fish/group) were released. Half of the fish of each group were tagged with an external tag with the message “reward” and the other half of the fish were implanted with tags with the message “$100 reward.” These fish were released into two estuaries in each state (n=4); three replicate groups were released at different sites within each estuary (n=12). From results obtained in previous tag return experiments conducted by wildlife and fisheries biologists, it was hypothesized that reporting would be maximized at a reward level of $100/tag. Reporting level for the “reward” tags was estimated by dividing the number of “reward” tags returned by the number of “$100 reward” tags returned. The cumulative return level for both tag messages was 22.7 (±1.9)% in SC and 25.8 (±4.1)% in GA. These return levels were typical of those recorded by other red drum tagging programs in the region. Return data were partitioned according to verbal survey information obtained from anglers who reported tagged fish. Based on this partitioned data set, 14.3 (±2.1)% of “reward” tags were returned in SC, and 25.5 (±2.3)% of “$100 reward” tags were returned. This finding indicates that only 56.7% of the fish captured with “reward” tags were reported in SC. The pattern was similar for GA where 19.1 (±10.6)% of “reward” message tags were returned as compared with 30.1 (±15.6)% for “$100 reward” message tags. This difference yielded a reporting level of 63% for “reward” tags in GA. Currently, 50% is used as the estimate for the angler reporting level in population models for red drum and a number of other coastal finfish species in the South Atlantic region of the United States. Based on results of our study, the commonly used reporting estimate may result in an overestimate of angler exploitation for red drum.

Movements of bocaccio (Sebastes paucispinis) and greenspotted (S. chlorostictus) rockfishes in a Monterey submarine canyon: implications for the design of marine reserves

In August and September of 1997 and 1998, we used SCUBA techniques to surgically implant Vemco V16 series acoustic transmitters in 6 greenspotted rockfish (Sebastes chlorostictus) and 16 bocaccio (S. paucispinis) on the flank of Soquel Canyon in Monterey Bay, California. Fish were captured at depths of 100–200 m and reeled up to a depth of approximately 20 m, where a team of SCUBA divers anesthetized and surgically implanted acoustic transmitters in them. Tagged fish were released on the seafloor at the location of catch. An array of recording receivers on the seafloor enabled the tracking of horizontal and vertical fish movements for a three-month period. Greenspotted rockfish tagged in 1997 exhibited almost no vertical movement and showed limited horizontal movement. Two of these tagged fish spent more than 90% of the time in a 0.58-km2 area. Three other tagged greenspotted rockfish spent more than 60% of the time in a 1.6-km2 area but displayed frequent horizontal movements of at least 3 km. Bocaccio exhibited somewhat greater movements. Of the 16 bocaccio tagged in 1998, 10 spent less than 10% of the time in the approximately 12-km2 study area. One fish stayed in the study area for about 50% of the study time. Signals from the remaining 5 fish were recorded in the study area the entire time. Bocaccio frequently moved vertically 10–20 m and occasionally displayed vertical movements of 100 m or greater.

Intermingling and seasonal migrations of Greenland halibut (Reinhardtius hippoglossoides) populations determined from tagging studies

A total of 7244 Greenland halibut (Reinhardtius hippoglossoides, Walbaum) were tagged in Greenland waters between 1986 and 1998 to increase information on stock delineations, to clarify migration routes, and to describe the seasonal movements of fjord populations. At present 517 recaptured Greenland halibut have been recorded. For Greenland halibut released in Davis Strait, Baffin Bay, and the fjords of southwestern and eastern Greenland, a substantial portion of recovered fish demonstrated migratory behavior, up to 2500 km, primarily to Denmark Strait between Greenland and Iceland. The recaptured fish provided evidence of intermingling between the population in Denmark Strait and the populations in Davis Strait and the southwest Greenland fjords. These observations support those of other studies that indicate that Greenland halibut inhabiting Davis Strait and the fjords of southwestern and eastern Greenland originate in the spawning grounds west of Iceland. The high mobility of offshore Greenland halibut within Baffin Bay and Davis Strait suggests that Greenland halibut migrate extensively between feeding and spawning areas. Greenland halibut in the fjords of northwestern Greenland appear to be resident in behavior and do not intermingle with offshore or more southerly inshore populations. A seasonal pattern in the recovery of these fish indicates that Greenland halibut aggregate in the inner part of fjords during the second half of the year (when inshore waters are not covered with ice).

Movements, behavior, and habitat selection of bigeye tuna (Thunnus obesus) in the eastern equatorial Pacific, ascertained through archival tags

Ninety-six bigeye tuna (88– 134 cm fork length) were caught and released with implanted archival (electronic data storage) tags near fish-aggregating devices (FADs) in the equatorial eastern Pacific Ocean (EPO) during April 2000. Twenty-nine fish were recaptured, and the data from twenty-seven tags were successfully downloaded and processed. Time at liberty ranged from 8 to 446 days, and data for 23 fish at liberty for 30 days or more are presented. The accuracy in geolocation estimates, derived from the light level data, is about 2 degrees in latitude and 0.5 degrees in longitude in this region. The movement paths derived from the filtered geolocation estimates indicated that none of the fish traveled west of 110°W during the period between release and recapture. The null hypothesis that the movement path is random was rejected in 17 of the 22 statistical tests of the observed movement paths. The estimated mean velocity was 117 km/d. The fish exhibited occasional deep-diving behavior, and some dives exceeded 1000 m where temperatures were less than 3°C. Evaluations of timed depth records, resulted in the discrimination of three distinct behaviors: 54.3% of all days were classified as unassociated (with a floating object) type-1 behavior, 27.7% as unassociated type-2 behavior, and 18.7% as behavior associated with a floating object. The mean residence time at floating objects was 3.1 d. Data sets separated into day and night were used to evaluate diel differences in behavior and habitat selection. When the fish were exhibiting unassociated type-1 behavior (diel vertical migrations), they were mostly at depths of less than 50 m (within the mixed layer) throughout the night, and during the day between 200 and 300 m and 13° and 14°C. They shifted their average depths in conjunction with dawn and dusk events, presumably tracking the deep-scattering layer as a foraging strategy. There were also observed changes in the average nighttime depth distributions of the fish in relation to moon phase.

玉米品种更替中光合效率与产量提高的生理生态机制

本文以我国不同年代推出的玉米品种为试验材料，深入研究玉米品种更替中产量提高和光合效率改良的演变特征及生理机制，以及环境胁迫影响各年代玉米品种光合效率及产量的生理机制，在此基础上分析光合改良与产量提高的关系。 对各年代玉米品种花后光合效率的变化及其与产量形成的关系的研究发现：虽然新品种的光合速率在开花前并不比老品种高，但是它们能够迅速扩大叶面积，形成较多的籽粒，为接下来的生长准备了比较大的源和库。花后茎秆的功能由‘库’转为了‘源’，即能够从茎秆中转移一些同化物到籽粒中。而且新品种能够在花后维持较高的光合速率，衰老较慢，光合有效期较长，这样就能够为籽粒灌浆提供较多的新同化产物。因此新品种具有很强的调节光合器官功能和优化分配同化产物的能力，能够使各部位叶片的光合生产，植株的生长和发育以及干物质的分配与再分配都有利于籽粒产量的形成。新品种在花后能够维持较高的光合活性主要是因为它们能够在老品种叶片可逆性衰老的时候维持较高的叶绿素含量和可溶性蛋白质含量，从而未像老品种那样进入不可逆转的衰老阶段。对各年代玉米品种光合特性日变化的研究也发现新品种的光化学效率比较高。老品种光合作用的午间降低是由严重的光抑制引起的。 对于N素缺乏和水分胁迫对各年代玉米品种产量及光合效率的影响机制的研究发现新品种一方面能够在胁迫条件下维持较高的光合速率，另一方面仍能维持较长的光合有效期。即新品种能够在胁迫条件下维持较高的光合生产能力，从而能够形成较大的生物产量和籽粒产量。新品种在缺N情况下能够保持较高的光合活性是因为它们能够维持较高的PEPCase活性、叶绿素含量和可溶性蛋白质含量，也就是说新品种能够在缺N情况下维持叶绿体结构组分的完整性和功能性，从而维持较高的光合效率。而玉米新品种在水分胁迫时能够保持较高的光合活性是因为它们能够在水分胁迫时维持较高的叶片水势，使光合器官的功能得到良好的保护，复水时光合也能够比较迅速和完全的恢复。

玉米高产优质生产中若干生理生态问题的研究

玉米（Zea mays L.）是我国十分重要粮食、饲料和工业原料作物，种植区域覆盖我国大部分农业区。随着玉米品种改良和新栽培技术的应用，我国玉米产量大幅度增加。自1950s以来，我国玉米产量递增幅度为126kg/hm2/yr。在玉米产量提高过程中，单叶光合作用与产量之间存在什么样的关系？当代玉米品种的品质和养分利用效率如何？高密度种植条件下是否存在“根系拥挤”及如何调控等。为探讨上述科学问题，本研究选择中国北方常见的大田玉米品种，在高肥力自然光照条件下，探讨玉米高产优质栽培过程中生理生态特征的变化趋势，以指导科学育种和栽培。主要研究结果如下： 　　1）光合与产量的演变我国 1950s、1970s、1990s等不同年代推广的玉米品种中，当代品种叶片光合速率高且高值持续期长，光合色素叶绿素a、叶绿素b、类胡萝卜素等的含量高且持续时间长，与光合有关的蒸腾速率（E.）、细胞间隙CO2浓度（Ci.）、气孔导度（gs）等也有较大改良，中下部叶片尤其明显;在生育后期，当代品种具有更高的光合优势。老品种饱和光合速率(Psat)在灌浆期下降，并非RuBPCase 和PEPCase的活性降低，而是由于叶绿素含量和可溶性蛋白含量的降低。在花后期间，由于PS2功能的下降，造成了光合能力下降，而现代品种的PS2 功能在衰老前一致保持旺盛状态。 　　老品种光合特征对缺氮的反应表现更敏感。花后缺氮光合作用下降是非气孔限制的，因为气孔导度和胞间CO2浓度没有发生明显的变化。其主要原因是缺素造成老品种叶片早衰，叶绿素含量、可溶性蛋白含量、PEP羧化酶活性下降。现代品种表现较强的抗衰老能力，其N素利用效用高于老品种。我国玉米产量的大幅度提高在很大程度上应归功于叶片光合性能的改良。 　　随玉米品种更替，群体光合速率增强，群体光合衰减率降低，呼吸消耗所占总光合的百分率下降。灌浆期当代品种中下部叶片的群体光合速率明显高于老品种。种植密度是影响玉米群体光合速率的主要因素，在高中低三种密度条件下，当代品种均有较高的群体光合速率，表现出耐密性强、适应性广、源足库大、产量高的特点。 　　2）高油玉米的产量受到叶源大小和叶源活力的双重限制在 1.5 株/m2密度下，与普通玉米相比较，高油玉米单株籽粒产量显著低于普通玉米，产量构成中穗粒数差异不显著，千粒重较低（P＜0.01）;两类型玉米的单株库容量相当，高油玉米籽粒灌浆速率小，籽粒充实度低，单粒重对叶源相对减少（剪叶）或相对增多（疏库）的反应比普通玉米更为敏感，其产量受到同化产物供应（叶源）相对不足的限制。高油玉米授粉后的叶面积、叶面积持续期小，叶片含氮量和光合速率较低，说明高油玉米的产量受到叶源活力（光合速率）小和叶源数量少的双重限制。 　　3）我国北方玉米品种的个体产量潜力、氮素利用效率及籽粒与秸秆粗蛋白质含量在充分发挥个体生产潜力的低密度条件下，我国北方1990s 以来大面积种植的50个玉米主栽品种中，个体产量潜力和氮素利用效率高度正相关(P=0.01)，而子粒千粒重与NUE 呈显著性负相关(P=0.002)。对玉米产量和氮素利用效率进行分层聚类，可将北方玉米品种划分为高产高NUE 型、低产低NUE 型和中间型，高产高NUE 型玉米品种相对较少，仅占24%。籽粒粗蛋白质含量（CPC）与秸秆CPC 相关性不显著（P＞0.05）。对籽粒和秸秆的CPC 进行分层聚类，将北方玉米品种划分为籽粒高秸秆低型、籽粒与秸秆双低型和籽粒与秸秆双高型，CPC 双高型品种相对较少，仅占20%。 　　4）玉米根系拥挤效应对产量影响的生理生态机制及其调控随玉米品种更替根系的空间分布呈“横向紧缩，纵向延伸”的特点。当代三类型玉米根系分布特性与株型、穗型相关。紧凑型品种根系分布深，下层根系所占比率大，适合密植，群体产量潜力大;平展大穗型品种根量多，分布较浅，在低密度下可获得较高的个体生产力，但不适合密植，群体产量潜力小。 　　“根系拥挤”显著影响玉米产量，减小根系横向伸展空间，下层土壤中的根系分配比率增多。在地上部充分生长条件下，紧凑型品种横向空间为30-50cm即可满足要求，平展型品种大于50cm;紧凑型品种对纵向空间受限制的反应更为敏感，平展型品种对横向空间受限制的反应更为敏感。“根系拥挤”影响根系活性、分布、氮素吸收利用和花后光合与14C同化物的分配。 　　在根系受限制条件下，增施肥料产量提高，根系总重增加，增加了根系在深层土壤（60-100cm）中的根系比率，显著增加了根系的TTC 还原量、SOD、CAT、POD活性。土壤加沙，根量减少，但根系TTC 还原量增加、产量提高，提高幅度以大穗型品种更为显著。 　　随种植密度增加耕层根系密度与群体产量同步增大，各类品种均在最高根系密度下获得最高产量。根系负荷的籽粒产量潜力三类型品种存在极大差异，在一定范围内增大种植密度，根系伸展空间减小，群体产量提高，紧凑大穗型品种产量最高，品种的耐密性是限制根系负荷籽粒产量潜力的主导因素。因此，培育株型紧凑、耐密性强、大穗玉米良种，采取有效的调控措施是玉米进一步高产的主攻方向。 　　5）我国夏玉米高产田的培创理论研究与实践相结合，2005 年在我国华北地区的山东莱州培创出籽粒实产21 042.9kg/hm2 （ 14% 含水量， 实收面积=45.7m×15.9m=726.63m2）的夏玉米高产纪录。主要采用以增加密度为保障的“群体结构性挖潜”和以提高整齐度为保障的“个体功能性挖潜”途径，生理生态指标包括：选用紧凑抗倒耐密植品种DH3719，种植密度102 030 株/hm2，收获密度98 610 株/hm2，花后具有较长的叶面积高值持续期，达60d以上，叶面积指数最大为6.53，收获2.59。上部叶片光合值对外界光强度变化敏感，其光合峰值出现时间提前，而后迅速衰减;中部叶片光合值的降低较慢，下部叶片变幅最小，可能是长期处于争光环境表现出的生态适应性。粒叶比0.32，经济系数0.542，单株产量216g，千粒重375.1g。

Early and better maturity in carp brood stock with control of environment and feeding

Available of carp breeders in their prim state of maturity is a major constraint in hypophysation. Experiments conducted in a fish farm at Naihati, West Bengal, for two consecutive years, 1983-84 and 1984-85, clearly prove that by manipulation of environmental parameters such as metabolites, dissolved oxygen, running water conditions, as also of stock densities and quality and quantity of feed. Catla catla, Hypophythalmichthys molitrix, Labeo rohita, Cirrhina mrigala and Ctenopharyngodon idella can be made to attain better maturity and spawning stage much earlier than normal i.e. even in summer months and the entire stock spawned during the period from March to September. Percentage of successful breeding, quantities of eggs released and fertilised in relation to the body weight of all the species, were also found to be more in comparison to the brood stock raised through the conventional methods.

Studies on the filtration, feeding and excretion rates in Perna viridis, Marcia cor and Crassostrea gryphoides (Mollusca: Bivalvia) using P³² labelled Ankistrodesmes

The study deals with a series of experiments to investigate feeding and excretion in three species of bivalves: Perna viridis (Linné), Marcia cor (Sowerby) and Cassostrea gryphoides (Gould) from Manora Channel, Karachi. Bivalves were fed with suspensions of Ankistrodesmes labelled with P³². These animals showed a considerable variation in the average filtration rates depending upon species and the body lenght. Exceptionally high content of the P³² introduced with Akistrodesmes, got excreted as pseudofaeces and faeces within first three days following its absorption as a meal. The assimilated P³² is partly released as faecal material and its major proportion is directly transferred to the solution. As expexted the gonad and kidney are the main organs found responsible for excretion as comared to other body parts. Although, the assimilated P³² is mostly concentrated in the digestive glands, the results also show a significant presence of P³² in the gonads. Accumulation of P³² was the least in the foot.

Lunar, diurnal and tidal periodicity in relation to the prawn abundance and migration in the Godavari estuarine systems

Preliminary investigations to study the lunar, diurnal and tidal periodicity in abundance and migrations of prawns were made in the Bairavapalam distributary of the Goutami branch of the Godavari estuary during the period of November 1961 to July 1962. The study was based on observation of the catches of a stake net (bag net) operated near the mouth of the estuary. Records of the catches were maintained tidewise daily. Comparative estimates of abundance were made on the basis of catch per hour's operation. It was generally observed that the catches were higher during the darker half of the month than in the brighter fortnight. The landings during low tides were generally higher than those during high tides and usually heavier catches were made during nocturnal low tides than during the day low tides. A continuous inward and outward (immigration and emigration) movement of prawns of all size groups was observed in the estuary from November to July. In general, more penaeid prawns were found to be immigrating at dawn than at dusk. Similarly, the number of emigrants was also found to be generally higher during the new moon period than during the full moon. Metapenaeus monoceros showed an almost distinct nocturnal periodicity in migration, while no such periodicity was observed, distinctly, in other species. In the case of Penaeus indicus the movement of migrants was prolonged. In M. brevicornis the migrants were scarce till March and thereafter increased numerically. The migrant forms of Metapenaeus dobsoni continued to be abundant till May with peak periods in January and February. The migratory pattern of Metapenaeus affinis was similar to that of Metapenaeus brevicornis, though the migrants of the former species appeared a month earlier than the latter. Intensive studies over extended areas for longer periods are required to understand clearly the migratory pattern of the various species. The phenomenon of immigration of prawns can be clearly understood only by vital staining or tagging studies. Perhaps the emigrants might be returning with the succeeding changing life. To verify this, laboratory experiments, by vital stains, were conducted. The marked specimens, if released during the low tides on a large scale, may be recaptured during the subsequent high tides and the duration also may be calculated. At least some percentage of the emigrants remains in the sea for maturity and breeding.

On the operation of small shrimp trawls in shallow waters-scope-ratio and size-depth relation

Studies on small trawls seem to be comparatively less. These trawls are generally operated in shallower waters, where due to the limitations in the length of warp that could be released, size restrictions have to be considered for their efficient functioning. An attempt has been made to assess the effective scope-ratio of length of warp required for the operation of trawls at shallower depth and to a judge the size of trawl suitable for use at lower depths.