1. Introduction The hilsa shad (Tenualosa ilisha, locally known in Myanmar as Nga Tha Lauk) is a euryhaline clupeid distributed throughout freshwater, estuarine, and coastal regions of South Asia – from Kuwait in the Persian Gulf eastwards to Myanmar in the Bay of Bengal (Whitehead, 1985). Hilsa supports important commercial fisheries, particularly in Bangladesh, Myanmar, and India. It is fast growing, reaching sexual maturity within six to twelve months and living for up to six or more years (Milton, 2009; Rahman & Cowx 2006). Hilsa is typically understood to be anadromous, migrating from marine to freshwater for spawning, but it also demonstrates amphidromy, whereby immature fish migrate between fresh and marine waters for non-spawning purposes (Rahman et al, 2018). Spawning migration is thought to take place in large numbers with the onset of the southwest monsoon and associated flooding, when water depth, current velocity, volume of discharge stimulating flood pulse and temperature are favourable (Ahsan et al, 2014; Bhaumik, 2015a). However, movements are complex and varied, and some permanent freshwater and marine stocks have also been observed (Bhaumik, 2015a). Within the Bay of Bengal, otolith microchemistry and allozyme variation provide evidence of substantial gene flow between groups of hilsa, indicating that fish in Myanmar may also have spawned in India or Bangladesh, and vice versa (Milton & Chenery, 2001; Salini et al, 2004). In Myanmar, the hilsa is distributed throughout the Ayeyarwady Region and adjacent Rakhine State, migrating between rivers and the coastal zone (Baran et al, 2015). Owing to its relatively high value (DoF, 2018), it is the main target species in these areas and a major source of income for vulnerable fishing communities (Khaing et al, 2019). Yet there is evidence that overfishing and habitat destruction are threatening the sustainability of Myanmar’s hilsa fishery. Exploitation rates (the proportion of mortality caused by fishing) have been found reach 0.7 in the Ayeyarwady region, well beyond sustainable levels (BOBLME, 2015). Hilsa are often targeted in Myanmar during spawning runs and juveniles are caught using small mesh in fishing nets, reducing recruitment (Baran et al, 2015; Khaing et al, 2019). Under the Freshwater Fisheries Law (1991), all open-access inland and inshore marine fisheries are closed from May to July to protect fish spawning and recruitment, although timings vary between locations depending on the time at which local target species are expected to be spawning. The catch and captivity of freshwater juveniles, spawning fish, and fish ready to spawn are also banned from May to August. These regulations were developed on the basis of research by organisations including the World Wide Fund for Nature (WWF), International Union for Conservation for Nature (IUCN), Flora and Fauna International (FFI), WorldFish, and bi-lateral and multi-lateral donors under the Myanmar Fisheries Partnership (Akester 2019). However, they were not designed specifically with the spawning and migratory behaviour of hilsa in mind and fishery closures are rarely enforced (Soe et al, 2018; Tezzo et al, 2019). Previous studies of hilsa throughout the Indo-Pacific have called for the protection of spawning and nursery grounds, a reduction in juvenile catches, and regulatory compliance to rebuild depleted stocks (BOBLME, 2012). To be effective, spatial and temporal fishery closures must be based on an understanding of the reproductive biology of hilsa, particularly size at first maturity, the onset and duration of spawning seasons, and the location of spawning grounds. Any closure must not only coincide with peak spawning seasons, but also be long enough to allow sufficient spawning. A great deal of research has been conducted on hilsa reproduction, particularly in Bangladesh and India (Amin et al, 2008; Ahsan et al, 2014; Haldar & Amin, 2005; Rahman & Cowx, 2006). However, the spawning behaviour of hilsa varies throughout its range and over time, according to fluctuations in rainfall, upstream runoff, sediment input, and variation in habitat types (Hossain et al, 2019). It is therefore essential that spawning behaviour is assessed – and monitored over the long term – within Myanmar.Studies in Myanmar have used local ecological knowledge from fishers to identify migration routes and spawning sites of hilsa (Baran et al, 2015; Ko Ko et al, 2019). Baran et al (2015) identified spawning sites in 15 out of 32 locations surveyed, with the largest and most important spawning site found in the Hinthada Township area, which is located 230 to 310 km from the sea. Local knowledge indicated that spawning takes place in March-April in the majority of spawning sites identified, but further investigation was recommended.The aim of this paper is to interrogate existing understanding of spawning behaviour in hilsa, in order to determine the optimum timing for fishing ban(s). We assess seasonal and spatial patterns in the sex ratio, maturity, gonadosomatic index, and length-weight relationship of hilsa – parameters which, together, can be used to assess spawning behaviour and provide a robust basis for effective fishery management.The paper is structured as follows: first we describe the methods used for sampling the fish and collecting and analysing their data, then we present and discuss the results, before providing conclusions and policy recommendations.2. Methods2.1 Study sitesThrough consultation with township-level (the third tier of administrative hierarchy) fisheries managers from the Department of Fisheries (DoF) of the Ministry of Agriculture Livestock and Irrigation (MoALI) of Myanmar, we selected nine study sites from the Ayeyarwady Region on the basis of their importance as habitats for hilsa and accessibility of landing sites (Fig. 1). Although there are seasonal variations during dry and wet seasons, each of these sites falls within one of three ecological zones: freshwater, brackish water, or saline. Two freshwater sites were selected from the Ayeyarwady River system in Hinthada and Danuphyu townships. Although there is anecdotal evidence that hilsa have been seen as high as the Chindwin-Ayeyarwady River intersection at latitude 21.50, local fishers tend to view these townships as the northern-most hilsa fishing sites. Hinthada was described as a shallow seasonally flooded nursery area for hilsa and an important seed collection site for commercial inland aquaculture. Three brackish water sites were selected from Ngapudaw, Pathein and Maubin townships – areas of large and small rivers connected to the Ayeyarwady River that experience tides and saltwater intrusion. Maubin is thought to be important for spawning hilsa due to the convergence of the Maubin, Yangon and Toe Rivers. Four sites were selected from the coastal saline zone in Hainggyi, Labutta, Mawlamyinegyun and Pyapon townships. These are important fishing grounds for hilsa all year round. 2.2 Sampling and dissectionA team of fisheries scientists from the University of Yangon sampled a total of 8,793 hilsa fish from catch landed by fishers on a monthly basis from November 2017 to November 2018 – averaging 676 per month (Table 1). They sampled the fish at approximately equal intervals during the last week of each month. Weight and length measurements were recorded for each specimen. Of these, 982 (an average of 75 specimens per month) were randomly selected and dissected to determine their sex. For most of these specimens (942), gonad weight and length were also recorded. 519 of specimens for dissection were purchased, washed, stored on ice (with a fish to ice ratio of 1:2 by weight), and transported to the laboratory of the University of Yangon’s Zoology Department within a 36-hour period. On arrival at the laboratory, each fish was dried with a paper towel before weighing, measuring, and dissection. Due to budget restrictions, the rest of the specimens were examined at the collection site, before returning to sellers.