Aspects of the washout of salmonid eggs. 1. Observations on the physical characteristics of real and artificial eggs, their rate of fall in a water column and their pattern of settlement in an experimental channel

When salmonid redds are disrupted by spates, the displaced eggs will drift downstream. The mean distance of travel, the types of locations in which the eggs resettle and the depth of reburial of displaced eggs are not known. Investigation of these topics under field conditions presents considerable practical problems, though the use of artificial eggs might help to overcome some of them. Attempts to assess the similarities and/or differences in performance between real and artificial eggs are essential before artificial eggs can validly be used to simulate real eggs. The present report first compares the two types of egg in terms of their measurable physical characteristics (e.g. dimensions and density). The rate at which eggs fall in still water will relate to the rate at which they are likely to resettle in flowing water in the field. As the rate of fall will be influenced by a number of additional factors (e.g. shape and surface texture) which are not easily measured directly, the rates of fall of the two types of egg have been compared directly under controlled conditions. Finally, comparisons of the pattern of settlement of the two types of egg in flowing water in an experimental channel have been made. Although the work was primarily aimed at testing the value of artificial eggs as a simulation of real eggs, several side issues more directly concerned with the properties of real eggs and the likely distance of drift in natural streams have also been explored. This is the first of three reports made on this topic by the author in 1984.

Interactions of age-dependent mortality and selectivity functions in age-based stock assessment models

The natural mortality rate (M) of fish varies with size and age, although it is often assumed to be constant in stock assessments. Misspecification of M may bias important assessment quantities. We simulated fishery data, using an age-based population model, and then conducted stock assessments on the simulated data. Results were compared to known values. Misspecification of M had a negligible effect on the estimation of relative stock depletion; however, misspecification of M had a large effect on the estimation of parameters describing the stock recruitment relationship, age-specific selectivity, and catchability. If high M occurs in juvenile and old fish, but is misspecified in the assessment model, virgin biomass and catchability are often poorly estimated. In addition, stock recruitment relationships are often very difficult to estimate, and steepness values are commonly estimated at the upper bound (1.0) and overfishing limits tend to be biased low. Natural mortality can be estimated in assessment models if M is constant across ages or if selectivity is asymptotic. However if M is higher in old fish and selectivity is dome-shaped, M and the selectivity cannot both be adequately estimated because of strong interactions between M and selectivity.

Influence of shelf-break fronts on shellfish and fish stocks off Argentina [Contribution to ICES Annual Science Conference, Reykjavik, Iceland, 27 Sep- 04 Oct 1996]

The general circulation pattern in the western boundary of the SW Atlantic is dominated by the opposite flows of Malvinas (-Falkland)and Brazil Current. In the Confluence region both currents separate from the continental slope and flow offshore creating an area of strong contracts and complex dynamics. The shelf-break fronts off Argentina mark the transition between shelf waters of mixed origin and nutrient rich Malvinas waters. Two areas deserve special attention due to the steep gradients introduced by the outflow of important sources of continental waters: the Rio de la Plata and the Magellan Strait to the north and south of the study area. Characteristics of the front is the high primary and secondary production, and the presence of important invertebrate and fish stocks that concentrate along the front to feed or spawn. The area comprises nearly 30 o/o (333 million U$S in 1995)of all Argentine catches of fish and squid. Resources in the area, beyond the EEZ limits, support international fisheries mainly of Russia, Poland and Spain. (Document contains 15 pages & figs)

Fish and fisheries of Bakolori Reservoir

Details are given of fisheries activities carried out in Bakolori Reservoir within the context of many and varied functions of Sokoto Rima River Basin Development Authority. Experimental fishing was conducted for 55 days in 1979 when the reservoir was only one year old and the exercise continued in 1980 and 1981 also for 85 and 52 days respectively. During the exercise, the catch per unit effort showed an increasing trend with increasing efforts by four times to that of the initial. The fish harvest was 1.205 kg/day when 3.8 nets were operated in 1979, this went up to 3.2 kg when the number of nets were increased to 8.37/day in 1980. This increasing trend continued in 1981 season also when 16.15 nets caught 5.756 kg fish per day. A direct relationship was observed in the fishing efforts and catch. Tilapia, Clarias, Labeo, Schilbe and Synodontis spp, were the most predominant fish species in the catch and contributed more than 97% of the total fish harvest

Growth of skipjack, Katsuwonus pelamis, and yellowfin, Thunnus albacares, tunas in the Eastern Pacific Ocean, as estimated from tagging data

ENGLISH: The average linear growth rate of skipjack in the eastern Pacific is less than 1 mm per day except for fish 375 to 424 mm in length at release. The growth rate shows a decrease with increasing length and increasing time at liberty. The growth rate of fish in the length range of about 43 to 57 cm is apparently more rapid in the eastern Pacific than in the western Pacific. Dsing data for the northeastern and southeastern Pacific combined, K and ~ were estimated to be 0.658 (on an annual basis) and 885 mm, respectively, by the ungrouped method and 0.829 and 846 mm, respectively, by the grouped method. Sensitivity analyses have shown however, that the estimates of these parameters are poorly determined by the sum of squares method used to derive them. Estimates of K and ~ for the eastern Pacific tend to be lower and higher, respectively, than those for the western Pacific. The average linear growth rate of yellowfin in the eastern Pacific is a little less than 1 mm per day for fish between about 25 and 100 cm in length at release. The growth appears to be most rapid in Area 2 (Revillagigedo Islands) and slowest in Areas 1 (Baja California), 5 (Central America- Colombia), and 6 (Ecuador-Peru). There is considerable variation in the growth rates of individual fish. The growth does not show a decrease with increasing length or increasing time at liberty so realistic estimates of the parameters of the von Bertalanffy or other similar equations cannot be calculated from these data. If realistic estimates of these parameters are to be secured larger fish must be tagged and released or many more long-term returns from fish to about 100 cm in length at release must be obtained. The growth patterns for the eastern Pacific, central Pacific and eastern Atlantic found by most other investigators differ from one another and from those found in the present study. Some of these differences may be real and others may be due to deficiencies in the data or the methods of analysis. Estimates obtained from tagging data are believed to be realistic provided the tags do not inhibit the growth of the fish. It appears that the growth rates of single- and double-tagged fish are the same; this indicates, though not unequivocally, that the tags do not inhibit the growth. SPANISH: La tasa media de crecimiento lineal del barrilete en el Pacífico oriental es inferior a lmm/día, excepto en el caso de peces de entre 375y 424mm de longitud de liberación. La tasa de crecimiento disminuye a medida que aumenta la longitud y el tiempo en libertad. La tasa de crecimiento de peces de entre unos 43 y 57 cm de longitud parece ser mayor en el Pacífico oriental que en el occidental. A partir de datos del Pacífico nororiental y suroriental combinados, se estimaron K y loo en 0.658 (anual) y 885mm, respectivamente, usando el método no agrupado, y 0.829 y 846mm, respectivamente, usando el método agrupado. Sin embargo, los análisis de sensitividad han demostrado que el método de suma de cuadrados utilizado para derivar las estimaciones de estos parámetros las determina con poca precisión. Las estimaciones de K y loo para el Pacífico oriental suelen ser inferiores y superiores, respectivamente, a los del Pacífico occidental. La tasa media de crecimiento lineal del aleta amarilla en el Pacífico oriental es ligeramente inferior a lmm/día para los peces de entre unos 25y 100cmde longitud de liberación. El crecimiento parece ser más rápido en el Area 2(Islas Revillagigedo),y más lento en las Areas 1(Baja California), 5 (Centroamérica-Colombia), y 6 (Ecuador-Perú). Las tasas de crecimiento de peces individuales varían considerablemente. El crecimiento no muestra una disminuciónconun aumento en la longitud o en el tiempo en libertad, y por consecuencia no se se pueden calcular estimaciones realistas de los parámetros de la ecuación de von Bertalanffy u otras ecuaciones similares a partir de estos datos. Para obtener estimaciones realistas de estos parámetros sería necesario marcar peces mayores u obtener muchas más devoluciones a largo plazo de marcas de peces de unos 100cm de longitud de liberación. Los patrones de crecimiento correspondientes al Pacífico oriental, Pacífico central, y Atlántico oriental descubiertos por la mayoría de los investigadores son diferentes entre síy también de los del presente estudio. Es posibleque algunas de estas diferencias sean verdaderas, mientras que otras se deban a faltas en los datos on en los métodos analíticos utilizados. Se considera que las estimaciones obtenidas a partir de los datos de marcado son realistas, suponiendo siempre que las marcas no impidan el crecimiento de los peces. Parece ser que las tasas de crecimiento de peces con una marca y con dos son idénticas, lo cual indica, aunque sin certeza total, que las marcas no ejercen tal efecto. (PDF contains 76 pages.)

The relationships between fish assemblages and the amount of bottom horizontal beam exposed at California oil platforms: fish habitat preferences at man-made platforms and (by inference) at natural reefs

Between 1995 and 2002, we surveyed fish assemblages at seven oil platforms off southern and central California using the manned research submersible Delta. At each platform, there is a large horizontal beam situated at or near the sea floor. In some instances, shells and sediment have buried this beam and in other instances it is partially or completely exposed. We found that fish species responded in various ways to the amount of exposure of the beam. A few species, such as blackeye goby (Rhinogobiops nicholsii), greenstriped rockfish (Sebastes elongatus), and pink seaperch (Zalembius rosaceus) tended to avoid the beam. However, many species that typically associate with natural rocky outcrops, such as bocaccio (S. paucispinis), cowcod (S. levis), copper (S. caurinus), greenblotched (S. rosenblatti), pinkrose (S. simulator) and vermilion (S. miniatus) rockfishes, were found most often where the beam was exposed. In particular, a group of species (e.g., bocaccio, cowcod, blue (Sebastes mystinus), and vermilion rockfishes) called here the “sheltering habitat” guild, lived primarily where the beam was exposed and formed a crevice. This work demonstrates that the presence of sheltering sites is important in determining the species composition of man-made reefs and, likely, natural reefs. This research also indicates that adding structures that form sheltering sites in and around decommissioned platforms will likely lead to higher densities of many species typical of hard and complex structure.

Sea surface temperature and the subsequent freshwater survival rate of some salmon stocks: a surprising link between the climate of land and sea

Previous consideration of the relationship between climate and the survival rate of Pacific salmon eggs and fry has been confined to effects of large variation in the ambient freshwater environment; e.g., stream discharge, temperature, turbidity. This analysis shows sea surface temperatures during the last year of life of maturing adult salmon are also strongly associated with the subsequent survival rate of salmon eggs and fry is fresh water, presumably through development of the future eggs or sperm. In several stocks of three species of North American salmon, the association between the "marine" climate and egg survival is stronger than, or additive to, any estimated climatic association in fresh water. This apparent and surprising link between fresh water and the distant ocean has some interesting and complex implications for management of future salmon production.

An idealized rural coastal zone management integrating land and water use

Various countries have formulated special integrated coastal zone management (ICZM) strategies which seek to both manage development and conserve natural resources and integrate and coordinate the relevant people sectors and their functions and roles within the bounds of this rich realm. Concerns that may be addressed by ICZM include: 1) Natural resources degradation; 2) Pollution; 3) Land use conflicts; and, 4) Destruction of life and property by natural hazards. Some prevalent sources of environmental impacts (livelihoods) are listed, together with some recommendations to the concerns which they may raise in relation to coastal zone management: agriculture; aquaculture; fisheries; forestry; human settlements; tourism; and, transport industry.