Mortality index as an Indicator to Analyze Overall Selectivity

of a Multi-Species and Multi-gear Fishery


Lake Mweru –a multi-species and multi-gear piscary in Africa- has been sing with the addition in figure of attempt and variableness of stock over decennaries. It was assumed that in a multi-gear piscary with all fishing cogwheels that is selective in species-size and aim all species in entire fish community would take to the non-selective piscary that can conserve the comparative construction of fish community. It was hypothesized that the entire piscaries in Lake Mweru would bring forth an overall non-selectivity fishing form. Mortality index as a portion of life history features that correlated to maximum length was considered to be analyzed in order to see its alterations that reflecting the responses of fish community to fishing force per unit areas. The experimental information of 13-mesh size gillnets that collected from 1970s to 1990s were used and completed by garnering and categorising the maximal length informations. By utilizing the ANCOVA, the relationship between maximal length and mortality index was obvious to see how the mortality index of each species category alteration by maximal length over clip as the impacts of fishing force per unit areas. The mortality index analysis showed that the mortality index and the productivenesss addition for all species over decennaries whilst the addition of mortality index was more rapid for the big species. Therefore, the chief hypothesis which was stating the status of Lake Mweru can be bring forthing an overall non-selective piscary was partially accepted.

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Keywords: mortality index, maximal length, multi-gear, multi-species, non-selective piscary, fishing force per unit areas.


Lake Mweru is a extremely productive lake which has variable stock degrees of its of import species ( Jul-Larsen et al. , 2002b ) . Change in fish output is related to alter in lake productiveness ( Jul-Larsen et al. ( 2002a ) . The alterations of fish stock can be addressed to resilience constructs. Resilience is the capacity of a system to absorb perturbation and reorganise while traveling through with alterations so as to still retain basically the same map, construction, individuality, and feedbacks ( Walker et al. , 2004 ) . Capable to angle community, resiliency of species to fishing is defined as recovery capacity of a species after the population is depleted ( Feitosa et al. , 2008 ) .

To explicate the stock alterations, there are several force per unit area factors that should be taken into history. There are three elements need to be considered ( Jul-Larsen and Zwieten, 2002 ) . The first is the alterations in H2O degrees which are related to climatic alterations. The 2nd is the addition in angling attempt. The 3rd component is species resiliency and susceptibleness. Fishing can raise greater variableness in exploited populations and diminish resiliency to angling for several species every bit good. It is structurally and functionally threats the fish population and manifest straight in copiousness of mark species, habitat devastation and lessening in average size, indirectly to alter in fish community construction ( Hsieh et al. , 2006 ; Yemane et al. , 2005 ) . The alteration in community construction as fishing impact on ecosystem is due to the fact that angling cogwheels are selective in species and size ( Jul-Larsen et al. , 2002a ) .

The selectivity of angling cogwheel is of import for piscaries direction programme with the purpose of protecting the resource. On the other manus, the non-selective targeting fish is assumed by single-species perspective, as transporting the harmful or destructive and lead to growth-over-fishing. However, the selectivity besides raises much more job for industrialised piscaries subject to their discards and job specifying the “right” mesh size in multi-species piscary ( Jul-Larsen et al. , 2002a ) .

Refering to the selectivity of cogwheels in multi-species and multi-gear piscary, it is assumed that the fishing cogwheels are selective in species and size ( Jul-Larsen et al. , 2002a ) significance that one cogwheel catches a certain set of species over a certain length scope, while another cogwheel or the same gear employed and modified in the different methods besides catches another certain set of species with another length scope. However, if they are all together catch all the whole community, can we still name the piscary as “selective fishery” or can we call them as “non-selective fishery” ?

Harmonizing to Jul-Larsen et Al. ( 2002a ) , as respect to ecosystem point of position, if all fishing cogwheels target all species in entire fish community at rates proportional to their natural mortality form, they are non-selective piscary. They will conserve the fish community since the comparative construction of the ecosystem would be preserved and merely little parts of them would be reduced. The Figure 1 below shows how the non-selectivity piscary mechanism.

In order to cognize whether the piscary has selective or non-selective fishing form, the impacts of piscaries to the fish community need to be clarified. The impacts of piscaries can be explained by the species resiliency and susceptibleness. The susceptibleness of species to cogwheels can depict how the fish stocks react to fishing ( Stevens et al. , 2000 ; Jul-Larsen et al. , 2002a ) . To specify susceptibleness and exposure of fish community to fishing cogwheels, the analysis of life history features are developed ( Stobutzki et al. , 2001 ; O’Malley, 2010 ) .

In this survey, in order to cognize whether the stock variableness and entire piscaries in Lake Mweru will bring forth the selective or non-selective piscary, mortality index as a portion of life history features that correlated to maximum length was considered to be analyzed in order to see its alterations that represent the alterations and variableness of fish community as a response to fishing force per unit area ( Welcomme, 1999 ; Stobutzki et al. , 2001 ; King and McFarlane, 2003 ; Patrick et al. , 2009 ) .

Harmonizing to Milton ( 2001 ) , mortality index are used to be an appraisal of comparative mortality ( Z ) of each species. The species whose average length in the gimmick is closer to the smallest length caught are considered to hold a larger per centum of population removed and higher mortality. This research used the analysis of relationship between mortality index and theoretical upper limit length [ 1 ] performed over decennaries to see how the mortality index of each species change by maximal length over clip. From this analysis, the alterations in mortality rate of fish community in Lake Mweru were obvious to detect. Therefore, the purpose of this survey was to measure the alteration in mortality rate as one of sort life-history features of a fish community to fishing force per unit areas.

Materials and Methods

1. Materials

Lake Mweru is a freshwater lake on the longest side of Africa’s second-longest river, the Congo. It is located on the boundary line of Northern-Zambia and the Democratic Republic of Congo in the Luapula vale. This lake is a productive lake where production is significantly dependent on alimentary pulsation. In such a system, the development rate can be high. It potentially recovers fast and has high and variable output ; nevertheless the susceptibleness to increased fishing force per unit area is low ( Jul-Larsen et al. , 2002a ) .

The experimental fishing study gathered in Lake Mweru was conducted to supply the information over clip sing the alterations occur through alterations in angling attempt. The studies which were employed a fleet of multifilament gillnets runing from 25 millimeters to 178 millimeters stretched mesh with 13 millimeter additions were collected by the Department of Fisheries during 1970-1972, 1982-1985 and 1993-1999. The mesh sizes used for the experimental fishing were shown in Table 1.

2. Methods

2.1 Species classification

Speciess classification was done in order to see the alterations of the whole fish community in Lake Mweru where the 73 species were assembled into 26 groups by seting each species into the species classification mechanism. In the following measure, each species was specified into three maximal length classs that analyzed from informations distribution of all species. The groups of species were categorized as:

1. Small species ( L-max value was less than 31.06 )

2. Large species ( L-max value was more than 47.97 )

3. Medium species ( L-max value in between 31.06 and 47.97 ) .

2.2 Calculation of Mortality Index

Mortality index is related with recovery capacity. It was calculated by utilizing: theoretical upper limit length ( L-max ) of species that was collected; mean length of species from the gimmicks of each decennary ; and minimal length of species that was collected from gimmicks of overall decennaries. The index of mortality was calculated by the undermentioned expression:

Mortality index =( Lsoap– Lave)/ ( Lave-Lmin)

where Lsoapis the soap length a species, Laveis the average length at gaining control in the piscary, and Lminis the smallest length in the gimmick.

The closer the mean length is to the maximal length, the lower fishing mortality in the population. Mortality indicates the addition in angling attempt so that in this instance, the mean length of species in a population closes to the smallest length. This index was influenced by the old and current fishing attempt. Besides, this index is under premise that catchability and mortality is changeless across the whole length ranges caught ( Stobutzki et al. , 2001 ) .

2.3 The mortality index ( MI ) over theoretical maximal length ( L-max )

There were three analyses taken sing the relationship MI and L-max as the followers:

  1. To reply the inquiry whether MI addition over LM and to research the relationship of the mortality index and maximal length, the comparing between the separate inclines & A ; intercepts and the overall incline & A ; intercept were examined. Therefore, the full theoretical account appears as follows:

Myocardial infarctionij= a + b.LM + Decadeij+ cI.LM ( Decadeij) +?ij

Where MI = mortality index ;

LM = theoretical upper limit length ( centimeter ) ;

a= overall intercept ;

b= overall incline ;

i= the decennaries 1970s, 1980s, and 1990s ; and

j= the grade of freedom used.

  1. To analyse whether the additions occur each decennary, the analyses were focused on comparing intercepts of each decennary. The ANCOVA theoretical accounts were defined as:

Myocardial infarctionij= a +b.LM + Decadetwo+?ij

  1. To hold significance whether the additions differ per decennary, the analysis was performed by taking into history the old theoretical account. Therefore, the theoretical account was presented as follows:

Myocardial infarctionij= a+ b.LM + degree CelsiusI.LM ( Decadeij) +?ij

Harmonizing to this ANCOVA theoretical account, the separate inclines and the overall incline were compared.


The relationship of mortality index and maximal length over decennaries were taken to see how the mortality index of each species changed by maximal length over clip. To look into whether there was the systematic alteration in overall mortality index over the maximal length over decennaries, the arrested development line through natural log of mortality index and upper limit were performed in order to normalise the information as follows (10log-transformed values ) :

The graph above used three parametric quantities of additive arrested development for severally the 1970s, 1980s, and 1990s, they are as follows (10log-transformed values ) : intercept = -0.1789, -0.0631, 0.2048 ; slope= -0.0196, 0.2895, 0.5161 ; R2= 0.00033, 0.0726, and 0.1401. All of coefficient findings are comparatively little. Besides, the significances in inclines were available for 1990s. While for intercept, they were important in the 1970s and 1990s.

The information points used is usually distributed. The remainders of informations were tested by utilizing Shapiro-Wilk ( W ) trial for severally the 1970s, the 1980s and 1990s: 0.9706, 0.9663 and 0.9522. To turn out normalcy of the informations, these W values should be near to 1 otherwise it will take to the rejection of normalcy. Their p-value shows significance of each W-value.

Harmonizing to the graph above, it can be noticed that there are alterations of inclines and intercepts over decennaries. There is an addition of intercept over clip between 1970s and 1990s. These indicate that during those times the mortality index increased for both the smaller species and the bigger species ( the indicant of mortality index of smaller and bigger species are noted in Appendix 9 ) .

To look into the significances of the inclines and intercepts above, the stepwise analyzing a series of arrested development theoretical accounts were taken. These will analyze the decrease in variableness in arrested development theoretical accounts where inclines and intercepts were fitted. Subsequently, the overall decrease of variableness represented by incline and intercept analysis was presented as follow

From Table 2, the sum of variableness explained by the differences between inclines ( r2=0.032 ) and intercept ( r2=0.188 ) in the full theoretical account are comparatively low. The full theoretical account gave the consequence that all intercepts were significantly different, while the inclines were significantly different merely for 1970s and 1990s, with the 1990s holding both highest incline and intercept. In the incline analysis, the incline in 1970s and 1980s resembled to each other. These two inclines were significantly different compared to the ninetiess. The inclines significantly increased in 1990s.

All intercepts were important. It increased significantly over decennaries. This represented that the mortality index increased for all persons over decennaries. Meanwhile, the inclines analysis indicated that during 1970s and 1980s the alteration in mortality could non be shown, the addition of mortality occurred during 1980s to 1990s and it was more rapid for the big species.

Harmonizing to Pauly ( 1980 ) , there is a relationship between mortality and length along with growing and temperatures. In this survey, this relationship between mortality and length were examined. The consequences showed that there was an overall addition of mortality index over decennaries. This indicated that the fishing force per unit area were non merely targeted the bigger species, but besides the medium 1s. As stated by Welcomme ( 1999 ) , the mullti-gear piscary has the wider selectivity scope. Therefore, this piscary was able to work all fish gatherings. The consequence besides said that the celerity of mortality index was higher for the bigger species. Since: ( 1 ) the bigger species are more vulnerable to fishing force per unit area ; ( 2 ) the multi-gear and multispecies piscaries targets the bigger species ; and ( 3 ) the generative scheme of many bigger species is slow-growing ( low-Rmax ) , therefore it was logical to province that the mortality index increased for all species over decennaries and it was faster for the bigger species.

The addition of mortality occurs during 1980s to 1990s might be caused by the fishing activities that late more intensive in Lake Mweru. It is noted that the monolithic addition in angling attempt for gillnet piscary in Lake Mweru since the early 1990s ( Jul-Larsen et al. , 2002b ) . However, angling and species susceptibleness ( life-history indexs ) are non the lone one cause of these alterations. It might be besides consequence of the environmental variableness in Lake Mweru. The diminutions in entire gimmick rates are related with periods of highly low H2O degrees. The gimmick rates will stabilise when the H2O degree rise once more ( Jul-Larsen and Zwieten, 2002 ) .

To place the alterations of fish gatherings as response to fishing, there are many ways to be taken into history. One of them is the analysis of critical parametric quantity. Actually, there are still several life histories that can be considered, such as trophic degree ( trophic operation ) , age/length adulthood and generative schemes. Therefore, the contemplations from those indexs are required for farther analysis.


It was summarized that the mortality index increased for all species ( little –medium-large ) over decennaries where it was more rapid for the big species. Therefore, the chief hypothesis which was stating the status of Lake Mweru could be bring forthing an overall non-selective piscary was partially accepted. This can be seen from Figure 4.


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