Sexual condition of female Hilsa was sampled from October 1983 to August 1984 from freshwater, estuarine and marine localities in Bangladesh. At Chandpur, on the River Meghna, changes in gonadosomatic index showed that early monsoon (June), late monsoon (October), and winter (February) are the major spawning periods. Ages of juvenile Hilsa were determined up to six months old (12 cm long) by counting daily growth rings of otoliths. Hatching dates of juveniles collected in different months were back calculated from otolith readings, and confirmed the existence of the three spawning peaks. Migratory movements of juvenile Hilsa in the River Meghna were studied from samples obtained by experimental beach seining, by experimental drift gill netting, and by fishermen's gear. Length-frequency analysis of fish from beach seines and from fishermen's catch indicated juveniles less than 8 cm in total length inhabit shore and shallow river areas (<10 ft depth). When juveniles attain a larger size (>7 cm) they migrate to the deeper river (>10 ft depth) in January. After reaching 12 cm, all disappeared from the deeper river in May. Their migration from the river towards the sea was indicated by the direction of gilling in experimental nets, and by mean length analysis of juveniles collected in the upper and downstream areas of the River Meghna. / Science, Faculty of / Zoology, Department of / Graduate

Data were gathered in 1983 and 1984 on the Hilsa fishery in the Meghna River in Bangladesh, and on factors related to its seasonal variation, including effort as estimated by weekly counts of fishing boats, and landings as estimated by counts of fish offloaded at Chandpur. Historical data were also analyzed concerning factors influencing annual variations in Hilsa landings from the Padma River of the Ganges (1967-1974) and from the Indus River of Pakistan (1968-1982). Rail shipments were good indicators of landings. No significant relationship was observed between riverine Hilsa landings and fishing intensity. Rainfall, mean water level and maximum air temperature had no obvious influence on annual variations in seasonal timing of Hilsa runs in the Padma river of the Ganges. Rainfall two years earlier had a significant negative relationship with annual variation of Hilsa landings, in both the Padma River and in the Indus River. Mechanisms regulating this negative association between landings and rainfall may be that Hilsa are 1+ year old when they are first recruited to the fishery, and the survival of eggs and fry is minimum in years when there is a very high rainfall. In the Ganges and Indus rivers, annual landings were not closely correlated, and appear to be dependent in part on earlier rainfall conditions within each region. This dependency offers a means of rough forecast of annual harvest on the basis of previous rainfall data. / Science, Faculty of / Zoology, Department of / Graduate

The study of spatiotemporal dynamics of fish populations is important for both stock assessment and fishery management. I explored the impacts of environmental and anthropogenic factors on spatiotemporal patterns of fish populations, and contributed to stock assessment and management by incorporating the inherent spatial structure. Hierarchical models were developed to specify spatial and temporal variations, and Bayesian methods were adopted to fit the models. Yellow perch (Perca flavescens) is one of the most important commercial and recreational fisheries in Lake Erie, which is currently managed using four management units (MUs), with each assessed by a spatially-independent stock-specific assessment model. The current spatially-independent stock-specific assessment assumes that movement of yellow perch among MUs in Lake Erie is statistically negligible and biologically insignificant. I investigated whether the assumption is violated and the effect this assumption has on assessment. I first explored the spatiotemporal patterns of yellow perch abundance in Lake Erie based on data from a 27-year gillnet survey, and analyzed the impacts of environmental factors on spatiotemporal dynamics of the population. I found that yellow perch relative biomass index displayed clear temporal variation and spatial heterogeneity, however the two middle MUs displayed spatial similarities. I then developed a state-space model based on a 7-year tag-recovery data to explore movements of yellow perch among MUs, and performed a simulation analysis to evaluate the impacts of sample size on movement estimates. The results suggested substantial movement between the two stocks in the central basin, and the accuracy and precision of movement estimates increased with increasing sample size. These results demonstrate that the assumption on movements among MUs is violated, and it is necessary to incorporate regional connectivity into stock assessment. I thus developed a tag-integrated multi-region model to incorporate movements into a spatial stock assessment by integrating the tag-recovery data with 45-years of fisheries data. I then compared population projections such as recruitment and abundance derived from the tag-integrated multi-region model and the current spatial-independent stock-specific assessment model to detect the influence of hypotheses on with/without movements among MUs. Differences between the population projections from the two models suggested that the integration of regional stock dynamics has significant influence on stock estimates. American Shad (Alosa sapidissima), Hickory Shad (A. mediocris) and river herrings, including Alewife (A. pseudoharengus) and Blueback Herring (A. aestivalis), are anadromous pelagic fishes that spend most of the annual cycle at sea and enter coastal rivers in spring to spawn. Alosa fisheries were once one of the most valuable along the Atlantic coast, but have declined in recent decades due to pollution, overfishing and dam construction. Management actions have been implemented to restore the populations, and stocks in different river systems have displayed different recovery trends. I developed a Bayesian hierarchical spatiotemporal model to identify the population trends of these species among rivers in the Chesapeake Bay basin and to identify environmental and anthropogenic factors influencing their distribution and abundance. The results demonstrated river-specific heterogeneity of the spatiotemporal dynamics of these species and indicated the river-specific impacts of multiple factors including water temperature, river flow, chlorophyll a concentration and total phosphorus concentration on their population dynamics. Given the importance of these two case studies, analyses to diagnose the factors influencing population dynamics and to develop models to consider spatial complexity are highly valuable to practical fisheries management. Models incorporating spatiotemporal variation describe population dynamics more accurately, improve the accuracy of stock assessments, and would provide better recommendations for management purposes. / Doctor of Philosophy / Many fish populations exhibit complex spatial structure, but the spatial patterns have been incorporated into stock assessment only in few cases. A full understanding of spatial structure of fish populations is needed to better manage the populations. Stock assessment and management strategies should depend on the inherent spatial structure of the target fish population. There have been many approaches developed to analyze spatial structure of fish populations. In this dissertation, I developed quantitative models to analyze fish demographic data and tagging data to explore spatial structure of fish populations. Yellow perch (Perca flavescens) in Lake Erie and Alosa group including American Shad (Alosa sapidissima), Hickory Shad (A. mediocris) and river herrings (Alewife A. pseudoharengus and Blueback Herring A. aestivalis) in selected tributaries of the Chesapeake Bay were taken as examples. Fishery-independent data for yellow perch displayed spatial similarities in the central basin of Lake Erie. Distinct temporal trends were observed in relative abundance data for Alosa sp. in different tributaries of the Chesapeake Bay. Substantial yellow perch movement among the central basin of the Lake was observed in tagging data. Ignoring the inherent spatial structure may cause fish to be overfished in some regions and underfished in others. To maximize the effectiveness of management in all regions for fish populations, I highly recommend incorporating spatial structure into stock assessment and management such as the ones developed in this dissertation.