Condition and Health Indicators of Exploited Marine Fishes

Josep Lloret, Georgiy Shulman, R. Malcolm Love

• Publisher: Wiley-Blackwell
• Year: 2013
• ISBN:
  9781118752784
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Industria ittica e acquacoltura

This important and informative new book outlines and discusses details of the basic principles and methods that are central to any study of fish condition, from a fish ecology and fisheries biology perspective. Condition and Health Indicators of Exploited Marine Fishes describes the potential capacities of condition indicators, providing examples showing the use of these indicators to solve practical problems in connection with fish ecology and fisheries research. By focusing on wild fish populations, the book complements the increasing number of scientific works that are contributing to show how fish condition studies are key to reveal problems in marine aquaculture, the effects of pollution, fish disease, and the importance of fish in human nutrition and medicine.

Condition and Health Indicators of Exploited Marine Fishes provides a comprehensive introduction to the study of fish condition that will assist advanced undergraduate and postgraduate students, researchers and professionals, working in marine ecology and biology, fisheries biology, environmental sciences and fish pathology. All universities and research establishments where biological and environmental sciences, fisheries and aquaculture are studied and taught should have copies of this book on their shelves.

1 Description of condition indicators

Summary: Biologists have developed a wide range of morphological, biochemical and physiological metrics to assess fish condition. This chapter introduces all these indicators and analyzes the simple methods and criteria used to assess the condition of fish, from simple ­morphometric (weight–length) to morphophysiological (liver, gonad and mesenteric fat weights) indicators. Each method has its pros and cons, along with limits in their application, which are here detailed together with practical recommendations. The utility of each method is shown using examples from different marine fish species around the world.

Key words: Fulton, Le Cren, condition factor, relative weight, liver (hepatosomatic) index, perivisceral (mesenteric) fat index, digestive index

Biologists have developed a wide range of morphological, biochemical, and physiological metrics to assess fish condition and health. These metrics were originally used to quantify aspects of human health, but have also proven useful to address questions in life history, ecology, and resource management of game and commercial animals (Stevenson & Woods, 2006). Condition is an important descriptor of fish health. Fish in good condition are assumed to have larger energy reserves than poor-conditioned fish, as well as optimum health. Here, “fish health” refers to the maintenance of homeostasis, including the normal occurrence of life cycles (primarily growth and reproduction) and the preservation of abundance and productivity of populations (Hochachka & Somero, 1973, 1984, 2002; Nemova & Vysotskaya, 2004; Depledge & Galloway, 2005). The condition of fish can be assessed by a variety of criteria, ranging from simple morphometric indicators based on weight–length data and morphophysiological indicators based on liver and gonad weights (Fig. 1.1) or on mesenteric fat, to physiological and biochemical measures such as lipid or protein content, the concentration of hemoglobin in the blood, the concentration of myoglobin in red muscle, the color and volume of the bile, the enzymatic activity of the tissues, the pH of the muscle after death, and the content of other substances such as glycogen, glucose, lactate, and creatine phosphate (Shulman & Love, 1999). Each of these measurements of fish condition has its own set of advantages and limitations, depending on the objectives of the particular study. In some cases, samples have to be processed and analyzed in the laboratory, which requires varying degrees of time, specialized training, instrumentation and expense, as in the case of biochemical analyses. In other cases the determination of condition indicators may involve only simple fish length and weight measures. Overall, the choice of condition ­criteria should be based on the objectives of the particular study, the particularities of each species, population and life stage with regard to body distribution and dynamics of energy reserves, as well as an understanding of the different condition techniques, including a detailed examination of the properties of the dataset as well as available laboratory or ­sampling facilities and budget. In some cases, integration of the information provided by different morphometric, physiological, and biochemical condition indicators may better reflect the overall physiological condition of the fish. In this chapter, the most used condition indicators in fisheries science are explained.

Figure 1.1 Dissection of a female European hake (Merluccius merluccius) showing the liver and ovaries. Photo by Dolors Ferrer.

Some of the best techniques for analyzing fish condition are time-consuming and/or costly (e.g., lipid analysis with gravimetric methods), whereas other techniques that have lower precision are easy to carry out on board or in the laboratory with minimum equipment and cost (e.g., evaluation of morphometric indices). The expert will need to assess which is the best method to use in each case, taking into account the balance between precision, cost, and time. Indicators of the condition of exploited fish are similar to those used to measure condition in medicine and veterinary sciences. The aim of research in this topic is to identify indicators of fish condition that can define the best possible state of organisms and populations, and which can also signal and quantify deviations from it. Here we focus on these indicators that are most suitable and which have been widely used for the evaluation of fish condition for fisheries ecology and management purposes. We must keep in mind, however, that a series of indicators, rather than just one, gives much more information. For example, the protein content of cod muscle decreases during starvation only after the level of liver lipid has dropped below a certain critical value (Black & Love, 1986), so the extent of depletion can be realistically judged only by measuring both. The determination of muscle protein alone fails to detect the early stages of depletion, while liver lipids do not change further during a long series of subsequent stages.

Some authors have argued that, apart from the morphometric, physiological and biochemical indicators described in this book, it is important to establish autopsy-based assessment of the health and condition of fish (Goede & Barton, 1990; Leamon et al., 2000). Several infectious agents, such as viral, bacterial, fungal and parasitic infections, are known to severely affect fish condition, leading to sublethal or lethal effects (Goede & Barton, 1990) and therefore an empirical autopsy-based system of organ and tissue indicators would improve our knowledge regarding fish condition and also the impact of disease(s) on natural fish populations.

Even though simple condition indicators are not always capable of estimating the health status of a given species, Lloret et al. (2012) proposed that those such as morphometric and organosomatic (biometric) indicators (e.g., Le Cren, Fulton, hepatosomatic), and whenever possible total lipid content, are used as a first step for evaluating the amount of energy reserves in fish. This should not be a substitute for standard stock assessment methods but can provide additional information for determining the status of a given stock. Moreover, simple measures of parasite infection such as prevalence, intensity, and abundance could be evaluated (Lloret et al., 2012). For practical purposes, the authors proposed that monitoring could also include the macroparasites (i.e., those large enough to be seen with the naked eye such as cestodes and nematodes) but not the microparasites (e.g., protozoans, which are more difficult to detect) even though they may also have an impact on the condition and reproduction of fish (see for example Kramer-Schadt et al., 2010; Sitjà-Bobadilla, 2009). The monitoring of parasitism will provide therefore a further index of fish health. While no single measurement of fish health uniquely indicates a source of stress (Buckley, 1985), all these simple related energy reserve and parasitism indicators taken together could be used as an index of fish condition (health).

In particular, the analysis of fish health during critical life periods (e.g., prior to spawning or migration, or in early life stages) is important for detecting the effects on stock productivity and thus their availability to the fisheries (Lloret et al., 2012). Several studies suggest that the condition of spawners at or just before the time of spawning would be a better proxy for reproductive potential (Marshall & Frank, 1999). In addition, knowledge of the lipid content in fish species can further enhance our ability to determine the human health benefits of consuming these fish, particularly with regard to essential fatty acids (fish are often promoted as a healthy component of the human diet because of high levels of essential fatty acids). In the following sections of this chapter we focus on simple condition (morphometric and organosomatic) indicators.

1.1 Morphometric indicators

Morphometric condition indicators are the simplest indicators of energy storage in fishery species. They are constructed with simple weight and length data that can be easily obtained from surveys or commercial landings using minimum and affordable equipment such as an ichthyometer (Fig. 1.2) and scale. These indicators assume that heavier fish of a given length are in better condition (Jones et al., 1999). Therefore, they are based on the premise that a fish of a given species and length should weigh as much as a standard for its length, and variations from the standard are taken as an indication of the relative fitness of an individual. These morphometric condition indicators have been available since the early 1900s and have undergone an evolution in methodology (Murphy et al., 1991). They have remained popular tools because they are inexpensive, simplistic, and mostly non-destructive, and are easily calculated from historical datasets that describe the length and weight of individuals (Lambert & Dutil, 1997a; Pope & Kruse, 2001). However, their use has remained sometimes controversial. Blackwell et al. (2000), Pope and Kruse (2001), and Nash et al. (2006) provide thorough reviews of the history of condition factors, together with the controversies surrounding their analysis and interpretation. Stevenson and Woods (2006) argue that ­morphometric condition factors actually measure the shape (i.e., girth) of a fish rather than being a direct measure of the extent of energy reserves, and several reviews have highlighted the statistical deficiencies of morphometric condition indicators (e.g., Cone, 1989; Hayes & Shonkwiler, 2001). To overcome potential bias and errors, Froese (2006) gives several ­recommendations for the proper use and presentation of morphometric condition factors, including guidelines for data collection and analysis of weight–length relationships.

Figure 1.2 Fish length measurement using an ichthyometer. Photo by David Caballero and Núria Zaragoza.

For the determination of condition indicators based on length and weight, it is always preferable to use eviscerated weight instead of total weight because th...