I don't think that there are any metrics that prove his statement. It is incredibly far fetched, like saying: 'countries in which more citizens wear glasses are also countries in which cancer is the most prevalent'. -> 'nearsightedness causes cancer'.
Posting a graph with that statement doesn't give it any more validity .

You should also take into account that there will be less and less farmland due to buildings

But in this specific case I think we're confused about the word urbanisation, which means a percentage increase (people moving from rural to cities), but also a land usage increase for urban areas.

That all really depends how you define countryside and city. It's different all across the world. Even in English speaking countries it differs, with different definitions .

I actually live in a small city with no skyscrapers

AMSTERDAM — The percentage of migrant residents in the four largest Dutch cities, Amsterdam, Rotterdam, Utrecht and The Hague, increased sharply from 36 to 43 percent between 1995 and 2003, the Central Bureau of Statistics (CBS) said on Monday.

Neither the graph nor the story on your page explains how plastic affects the fish stocks. You jump from 'fish stocks are declining' and 'plastic is increasing' to 'fish stocks are decreasing because of plastic' without actually explaining how fish die because of it, or how many fish die because of it.

One of the major problems with the plastic is that the contaminants cause premature death in animals. They can be ingested, leading to a painful death, and other dangers include strangulation or slow starvation if wildlife becomes imprisoned in the plastic. There is also concern about animals using the islands as transportation. They can end up far from their usual habitat and find themselves in unfriendly waters.

There are different types of ways that plastic is dangerous for humans. Direct toxicity from plastics comes from lead, cadmium, and mercury. These toxins have also been found in many fish in the ocean, which is very dangerous for humans. Diethylhexyl phthalate (DEHP) contained in some plastics, is a toxic carcinogen. Other toxins in plastics are directly linked to cancers, birth defects, immune system problems, and childhood developmental issues. To learn more on effects of plastics on humans visit the Ecology Center

Other types of toxic plastics are BPA or health-bisphenol-A, along with phthalates (mentioned above). Both of these are of great concern to human health. BPA is used in many things including plastic bottles and food packaging materials. Over time the polymer chains of BPA break down, and can enter the human body in many ways from drinking contaminated water to eating a fish that is exposed to the broken down toxins. Specifically, BPA is a known chemical that interferes with human hormonal function.

Plastics in the ocean is apparently also a problem for whales: