7! Marine Ecology, Life Histories and Modelling for Management
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CSTP Technical Report 2016 7! Marine Ecology, Life Histories and Modelling for Management 7.1! Introduction 7.1.1! Aims and Objectives The overall aim of Task 7 was to describe marine ecology, life histories and population dynamics of sea trout in the Irish Sea and to model relationships with key environmental features in order to provide management advice. Specific objectives were to: •! Provide a data-base on ecology and life histories for future research. •! Describe and quantify relationships between environmental variables, population life histories and stock features, focussing on adult fish. •! Develop a conceptual model of the relationships in terms of life history optimisation, to develop a theoretical process basis to the population dynamics and modelling. •! Develop preliminary life-history based models of responses to environmental and other pressures that will be of use in fishery management. Sea trout, the sea-going form of brown trout (Salmo trutta L.), are abundant in rivers entering the Irish Sea and support many river rod fisheries and a smaller number of estuarine and coastal commercial fisheries (see Workpackage 2). Sea trout fisheries vary in quality, expressed through catches, fish size distribution, abundance and seasonal run timing. The monitoring and assessment of these features are central preoccupations of fisheries management, which seeks to ensure that fisheries are in acceptable condition and that the stocks reflect good environmental condition (cf Water Framework Directive) in freshwater and marine habitats. The use by sea trout of multiple environments: freshwater, transitional and marine, during their life cycle makes them potentially valuable bio-indicators. Life histories directly influence the properties of stocks and fisheries by controlling abundance through survival and size distribution, and seasonal run pattern though the age at maturation and return from the sea. Therefore knowledge of life histories and their responses to environmental factors has considerable significance for fisheries and for the maintenance of biodiversity expressed through the variety of trout life histories. In particular, anadromy, the life history strategy of spending the adult stage at sea before return to and breeding in rivers, is what gives rise to sea trout; but it has many variations with fundamental implications for the fisheries. However, the relationships between trout biology, ecology, life histories and environment, are still poorly described and understood, particularly at sea (e.g. Elliott et al., 1992; Milner et al., 2006). Task 7 aims to describe and analyse these relationships for the Irish Sea in ways that are useful to the fishery managers for managing stocks, or for explaining why variation between fisheries occurs. The Task examines specific issues of sea trout marine ecology and life history variation, but also draws on information from other tasks to assemble understanding and advice that apply across the variety of sea trout stocks around the Irish Sea. Throughout this account the focus is on the anadromous form, sea trout, because they represent a distinct morph of the brown trout, with fisheries that are managed through different administrative procedures from the resident form. However, it must be emphasised that sea trout are simply one part of the life history range of brown trout and where feasible and appropriate the trout population complexes of accessible (to the sea) rivers in their entirety are considered. In practice often that is 401 CSTP Technical Report 2016 not feasible and this dichotomy of life histories, termed partial migration (Chapman et al., 2102), presents significant problems for fisheries assessment and management of trout. The Task 7 chapter opens with definitions of life history terminology and an explanation of the rationale for the approach to the task. An overview of previous knowledge of life history variation in the Irish Sea is given. The chapter then describes stock status and trends around the Irish Sea, the results of the studies on stock sizes and variation in key life history features, with a focus on growth and survival and explores some effects of climate change. The marine ecology and biotopes section describes feeding at sea and the habitat features of sea trout marine life. Some facets of life history based models of fisheries are outlined and the approach to this on-going work is summarised. Conclusions and recommendations are given. 7.1.2! Trout life History Terminology There are many stages and variations in trout histories giving rise to a specific terminology and a labelling system that supports age and spawning schedules as revealed through scale reading. The life stage nomenclature used here is based largely part on that proposed by Allan and Ritter (1977). 0+ fry or parr: fish in their first year of post-emergent life. May refer to 0+ fry for the earliest stages (e.g. <1month old) and to 0+ parr, up to the time of their first winter check (normally evident by April/May of their second year. 1+, 2+,…n+ parr: fish in their second and consecutive years of freshwater life. Residents: broadly, brown trout that do not migrate into coastal waters. There is a continuum of downstream migration in brown trout depending on local circumstances (e.g. feeding opportunities and migration risk); with fish reared in small nursery stream migrating to main stem rivers, to lakes, to estuaries or to the sea. While these are all functionally inter-breeding brown trout, but the fish that migrate to sea are considered separate because of the major environmental and life history shift on moving into open sea zones. Smolts: young trout having adopted the morphological and physiological adaptations necessary for migration downstream to the sea. There are various classifications of these migrants (see Bagliniére et al., 2013) associated with different adult life history types. Post-smolts: sea trout at sea after smolting and before their first return to river or first sea winter. Whitling, Finnock, Juniors, Herling: these are four of many synonyms for the youngest group of post-smolt sea trout that return to rivers in the same year that they smolt, therefore before their first post-smolt winter. They may be mature or not and therefore may or may not contribute to egg deposition. They usually return later in the year (after July), because they typically smolt between March and May. Maiden fish: adults that have not yet spawned, found in the sea or returning to the river for the first time since leaving the river as smolts (finnock are therefore maidens). The term applies to both sexes. Kelts: sea trout in rivers that have spawned and are migrating downstream back to sea. The scale reading terminology follows the conventions set out originally by Nall (1993) and subsequently used by most workers (e.g. Harris, 2000). This is described in the CSTP scale reading 402 CSTP Technical Report 2016 manual (CSTP, 2010). As the project developed and some of the characteristics of sea trout marine growth and river return behaviour in the Irish Sea became clearer additional terms were created to reflect the ambiguities, for example Indeterminate Mark (IM) (see Section 3.10). A special section on scale reading and the role that this might play in routine fisheries assessment is included in this report. However, because scale formulae are used in the text as shorthand for age class, some examples are given below, with their life history meaning. The formula (n.n) has two parts (before the point is freshwater and smolt life, after the point is post smolt, adult reproductive life). The formula conveys: •! age in winters (numerical), based on count of annual “winter” checks •! spawning history (alphabetical, SM) and marine residence •! growth status (denoted by presence of absence of +). Most fish have +, meaning growth (generally termed “plus-growth”) is in progress. Rarely there may be no +, meaning that a winter check has just formed on the edge of the scale. + is also used in multiple returning fish to unambiguously indicate growth between winter or spawning checks. Note that checks are conventionally termed “winter” checks, on the grounds that in most fish growth ceases or slows in winter due to low temperatures; but in practice sea trout can form genuine annual growth checks well into spring or in early autumn and maintain growth during winter months. 7.1.3! Brief Explanation of Sea Trout Scale Formulae (adapted from the CSTP Scale Reading Manual) The various terms used to describe the life history characteristics of sea trout follow the international Standard Nomenclature proposed by Allan & Ritter (1977). The conventions used to ascribe a numerical formula to describe the pre-smolt and post-smolt life history stages of sea trout are based on those adopted by Nall (1930), Went (1962) and Harris (2000). For example, a two year old smolt having returned to freshwater in the same year as its smolt migration is denoted as 2.0+, the point (.) indicating the separation between river and marine life phases. The initial number in the scale formula after the point records the number of complete post-migration winters spend at sea. In the CSTP, the convention has been adopted of referring to sea growth in first year of marine life as .0+. Although this is not always used in scale formulae of other accounts it provides an unambiguous statement of experience prior to the first spawning mark (SM) in the case of whitling. The + symbol serves two purposes; first it is a representation of the time between maiden sea growth and first spawning, second it represents a period of uncompleted summer growth between one winter and the next. The term maiden refers to a fish that has returned to the river for the first time after migrating to sea and spending a winter at sea as a post-smolt. B type smolt growth is additional growth after the final freshwater winter, associated with in-river growth during the downstream smolt migration, also known as "runout".