Candy Fish Experiment Write Up PDF

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ERSC 4240H – Fisheries Assessment & Management Workshop #4 Assignment – Candy Fish Experiment Submitted by Andrew Hodgson Thursday, March 1st, 2018 1.

Figure 1: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 3 fish per year fixed harvest yield. This fixed harvest yield does not display significant trends within the data, as all colours remain relatively stable in abundance and only fluctuate slightly over time.

Figure 2: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 13 fish per year fixed harvest yield. This fixed harvest yield displays a negative trend in that all colours decrease in abundance steadily over time.

Figure 3: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 23 fish per year fixed harvest yield. This fixed harvest yield displays a substantial negative trend for all colours, eventually reaching an abundance of 0 in year 5 of all colours.

Figure 4: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 33 fish per year fixed harvest yield. This fixed harvest yield displays a very strong negative trend for all colours, reaching an abundance of 0 in year 3 of all colours. The level of harvest which maximized sustainable yield in the fixed yield scenario was 3 fish per year in Figure 1. Firstly, this harvest level was the only one which saw an increase in one colour’s population (red). In addition, the three other colours remained fairly stable with only small fluctuations in abundance after each year. It is evident that this harvest level would be the most beneficial to the sustainability of the entire fish population in the long term. The three other harvest levels of 13, 23, and 33 fish exemplified over harvesting and the populations would eventually die out. This is shown through the negative abundance trends in Figures 2, 3, and 4.

2.

Figure 5: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 0.25 per year fixed harvest rate. This fixed harvest rate displays a negative trend over time for the blue, green, and yellow populations, whereas the abundance of the red population increases over time.

Figure 6: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 0.35 per year fixed harvest rate. This fixed harvest rate displays a positive trend for the blue and yellow populations over time, as opposed to the negative trend shown in the red and green populations.

Figure 7: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 0.45 per year fixed harvest rate. This fixed harvest rate displays a negative trend in the red, blue, and green populations, whereas the yellow population increases in abundance over time.

Figure 8: Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 0.55 per year fixed harvest rate. This fixed harvest rate displays a negative trend in the blue, green, and yellow populations, whereas the red population increases in abundance over time. The level of harvest which maximized sustainable yield in the fixed harvest rate scenario was the 0.25 per year fixed harvest rate, shown in Figure 5. This was the only level of harvest which did not result in a population with an abundance of 0 after the 5 year time period, such as what occurred in Figures 6, 7, and 8. This means that there are more fish available to harvest for a longer period of time than any other level of harvest used in this scenario.

Harvesting with a fixed yield and harvesting with a fixed rate differ for the sole reason that they ultimately result in a very different proportion of fish being harvested each year. A fixed yield means that each year, only a specified number of fish may be harvested, disregarding whatever the initial population is at the start of each year. On the other hand, a fixed rate of 0.25 for example, means that of the initial population at the start of each year, only 25 percent of that population may be harvested. This results in different numbers of fish being harvested each year as the total population either increases or decreases. In the case of which harvesting method is the most sustainable and has the highest yield, it is revealed from the previous figures that using a high fixed yield or high fixed rate will ultimately cause the population to collapse rapidly. However, using a high fixed yield caused every single colour in the population to collapse, whereas a high fixed rate only caused the collapse of a maximum of 3 colours. At a low fixed yield, the population was sustainable but the yield was poor, as seen in Figure 1. On the other hand, a low fixed rate resulted as well in a sustainable population, but with a larger yield. This is seen in Figure 5. Taking all of this into account, harvesting with a fixed harvest rate is the most sustainable while maintaining a higher yield than its counterpart.

3.

Figure 9: Group 1 Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 12 fish per year fixed harvest yield. This fixed harvest yield displays the same negative trend for the red and blue colours, as well as the same negative trend for the green and yellow colours.

Figure 10: Group 2 Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 12 fish per year fixed harvest yield. This fixed harvest yield displays a negative trend for blue, green, and yellow, whereas the red colour increases substantially in abundance over time.

Figure 11: Group 3 Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 12 fish per year fixed harvest yield. This fixed harvest yield displays a negative trend for red, green, and yellow, whereas the blue colour increases substantially in abundance over time.

Figure 12: Group 4 Abundance (# at start of year) over Time (Years) for an initial population of 50 fish with a 12 fish per year fixed harvest yield. This fixed harvest yield displays a negative trend for green and yellow, no trend for blue, and a positive trend for red as it increases in abundance over time. The trends in abundance of each colour of fish over time do differ from scenarios 1 and 2. In one sense, in almost every single case at least one colour (species) of fish ends up having an abundance of 0 within the 5 year duration of these models. However, the specific colours of fish which end up being reduced to 0 are different in most models. This is the case because, simply put, every scenario does not accurately assess the exact harvest yield or rate that is necessary to maintain a population while getting adequate yields. In addition, a difference is seen because scenarios 1 and 2 chose which colours were harvested at random, whereas scenario 3 had the harvesters choose which colours would be harvested. This scenario is analogous to a problem seen in real world fisheries where only certain species of fish are harvested at a time, leaving disproportionate numbers of other species to remain and overtake the competition for resources. Smaller populations of species cannot compete with much larger populations, specifically of predator species. The other problem with only harvesting certain species of fish in populations with a variety of different species is that eventually the harvested species will all be harvested, leaving only a few or a single species left in the ecosystem in question. This reduces the overall biodiversity of the ecosystem and will in turn reduce the productivity of the ecosystem. A problem such as this has been seen in cases such as the collapse of the cod fishery in the Maritimes. This problem could be prevented by attempting to estimate the total population of all species in the ecosystem, followed by harvesting an equal and sustainable amount of fish from all species included within the total population. This would maintain biodiversity and benefit the ecosystem in the long term.

4. a) Within the candy fish experiments conducted by Diaz Pauli and Heino (2013), ecological sustainability refers to if the harvesting method and rate proved to sustain the number of individuals in a population over time. For instance, poor ecological sustainability would mean that a large proportion of individuals

within the population became extinct during the duration of the experiment. On the other hand, evolutionary sustainability refers to if there is still biodiversity within the populations after the conclusion of the harvesting experiments. In this case, the best outcome would be that there was an equal proportion of all species left after the final harvesting year had occurred. b) The main factor within the candy fish experiments conducted by Diaz Pauli and Heino (2013) that influenced ecological sustainability was good governance occurring independently from the original experiment setup. For example, populations became more ecologically sustainable when participants began to manage which species would be harvested and the overall harvest rate. After the first round of the experiment, harvest rates declined substantially as the participants realized that this was the best way to ensure ecological sustainability at the end of the time period. This method of managing the harvest rate was most effective as it was an essential way of assessing the outcomes of the initial harvest and then adopting strategies to reduce these negative impacts. This good governance can be applied to fisheries practices in the context of real life issues.

Bonus Question: A process that occurs in wild exploited populations that affects population biomass and is not accounted for in the candy fish experiments is the mortality rate of species within a population. This could occur naturally (such as predation), or from human induced effects (reduced total oxygen content, pollution). The mortality rate would be applicable in this circumstance because the candy fish experiment included a recruitment rate after each harvesting year. The mortality rate would affect the number of fish within the population at the beginning of each year and therefore alter the results of the experiment....